Emergency services have been called to London's Liverpool Street Underground station after reports of an explosion.
The incident has caused major disruption to the entire network with stations across the capital closed.
A London Fire Brigade spokeswoman said four crews were at the scene and more were on their way. There are reports of "walking wounded".
Cordons are in place. A power failure may be behind the problems, Transport for London said.
[09.55 - Update from Geoff]
Fuke. Whole system has just been shutdown. Reports of trains colliding at Edwgare Road. There have been injuries. Keep your eyes peeled to the news people!
[10.40 - Update from Annie]
Police are confirming there was definitely an explosion at Russell Square - possibly in a litter bin. This is all hideous. It's hard to keep track of what is going on as no one knows whether it's power surges or what. But the official word is that terrorism cannot be ruled out. Just don't come into London today if you were planning to.
Blasts at least at Aldgate, Russell Square, Edgware Road and Liverpool Street. Fortunately no one reported killed but there are several injuries.
There have definitely been three explosions on buses. This is definitely something planned, it can't be power surges as was first claimed.
[11.00 - Update from Annie]
The Home Secretary Charles Clark has just made a very short statement basically saying there'll be more information when we know it. There have been terrible injuries, the worst in Russell Square and we're being advised not to make unneccessary journeys into town. And in fact I've just heard on the radio that all London buses have been suspended too. Mobile phone networks are overloaded so don't panic if you're checking that people are OK.
[11.20 - Update from Annie]
Everything that can be done is clearly being done by the emergency services. There are sniffer dogs around, police all over the place. I've just heard that the police are dealing with a suspect package at Stockwell station.
[11.30 - Update from Annie]
There have been 90 casualties at Aldgate station alone. Tony Blair is expected to make a statement at midday. The police are saying the situation is under control and all the emergency procedures that are continually rehearsed are being put into place. Six sites of explosions have been confirmed by the police.
[11.39 - Update from Annie]
Really sorry to report that two fatalities have been reported at Aldgate station.
[12.05 - Update from Annie]
I've just heard Tony Blair speaking, sounding very shaken and roughly he said: "It's reasonably clear there have been a series of terrorist attacks in London." He extended thoughts and prayers with the victims and their families. He's going to leave the G8 summit and return later this evening. "All leaders share our complete resolution to defeat terrorism.... It's reasonably clear to say this has been designed to coincide with the opening of G8....they will never succeed in destroying what we hold dear". Full text of Tony Blair's speech here.
I've just seen a picture of the Number 59 bus near Russell Square that was blown up and it's really horrible but shows the extent of the situation.
There is also a rescue operation going on at King's Cross tunnel.
[13.00 - Update from Annie]
More eyewitness reports are coming through from people who were on the tube or walking by the buses that were blown up. They all talk of the fear and panic, windows being broken, people praying and then gradually some people begin to take control and see what they can do to help the injured and get people out of the trains safely. Apparently there are still people Underground at Russell Square and King's Cross station who must be going through hell.
It's difficult to know what to write and say and all I can do is try to keep people updated as best I can. Most of the major news websites are down as they are flooded with traffic. Here are some pictures from the bbc's website.
Home Secretary Charles Clark is speaking in the House right now saying that first class care and support is provided by the emergency services. There are no buses in central London and no London Underground services. Airports are operating normally.
A group of Islamic terrorists called The Secret Organisation of al-Qaeda in Europe on a website today have apparently admitted to the explosions.
[13.15 - Update from Annie]
Tony Blair has just made another speech at Gleneagles as he's just about to leave. To sum up what he said: "All of our countries have suffered from the impact of terrorism. We are united in our resolve to confront and defeat this terrorism. this is not an attack on one nation but all nations..... We will not be allowed to stop the work of this summit.....The terrorists will not suceed. Today's bombings will not weaken our resolve....We shall prevail and they shall not."
[14.00 - Update from Annie]
I've just heard Mayor Ken Livingstone speaking very passionately from Singapore after yesterday's Olympic Win. My basic summary: "We have had trial emergency operations based on multiple explosions on the transport system and they are working well. I'd like to thank Londoners for the clam way they have responded to this attack. I have no doubt whatsoever that this is a terrorist attack. We did hope initially that this was a maintenance tragedy. this was not a terrorist attack against the mighty and the powerful this was aimed at ordinary working londoners. Irrespective of age, class, religion.
It is just an indiscriminate attempt at mass murder. Londoners will not be divided by this cowardly attack. They will stand together in solidarity around those that have been injured and those that have been bereaved. That is why I'm proud to be the mayor of this city. Speaking to the terrorists, I know that you personally do not fear to take your own life to take others. But you will fail. Look at the days that follow. People from around the world will arrive in London. They choose to come to London. Nothing you do, however many of us you kill will stop that flight to our city." Ken's full statement is here.
[14.15 - Update from Annie]
Just to trying to confirm where the explosions were. Four definitely confirmed attacks - One on a Metropolitan line train at Aldgate at the end of the line. One in the rear carriage of a Circle Line train between Paddington and Edgware Road. One on a Piccadilly Line train between King's Cross and Russell Square and the one where pictures are shocking everyone, on the Number 30 bus that was destroyed at Tavistock Square near Russell Square. It's still believed that two other buses may have been attacked and there were reports of explosions at King's Cross and Liverpool Street stations.
News is still coming in, but people are obviously going to be wondering how they will get out of Central London at the end of the working day. I've just heard that they're talking about re-opening bits of the overground at King's Cross.
The bbc's website seems to have recovered and there are some terrifying eyewitness accounts on it.
Traces of explosives have been found at two of the sites.
[15.20 - Update from Annie]
There is an emergency number if anyone is worried about people 0870 1566344 - please only call this in an emergency.
The fatality figures are changing all of the time from 20 to 45 but at the moment it's all speculation. However, looking at that bus I'm sure the figure must be higher than the two who were sadly killed at Aldgate.
Of the blogs and sites that are doing up to the minute reporting I would recommend the following
Brian Paddick Deputy Assistant Commissioner of the Metropolitan Police has just made an official statement:
There have now been 33 fatalities confirmed. Seven confirmed at Liverpool Street/Aldgate, 21 confirmed at Edgware Road and five at King's Cross/Russell Square. Numbers of fatalities for the bus have not been confirmed. However, there are walking wounded from the top deck of the bus.
No warnings were received of the attacks. The police have not received any statements. There's a news conference taking place now and Paddick is talking about CCTV footage being studied but there have been no arrests as yet. No one also knows whether a suicide bomber was involved.
Some buses are beginning to start running again in Zone one. People are bieng asked to stagger their home journeys. DLR has been suspended as a precaution and Victoria Coach station is operating a limited service.
Thames Clippers are putting on free travel on the river. Details on their website.
[16.10 - Update from Annie]
I've just watched some of the press conference with the Police and Tim O'Toole from London Transport. The police believe that the bombs were on tube train carriages themselves and not at stations. They don't believe that the bomb on the bus was intended to go off on the tube - which there's currently a lot of speculation about. There's likely to be another conference at the QE2 centre tomorrow morning.
Tim O'Toole said: "The reason it will be taking so long to get the services back is because we will be investigating every inch of every train. I can assure you there's no other metro system in the world that knows more about this than the London Underground."
[16.40 - Update from Annie]
A lot of people are leaving offices now in a staggered way rather than in a mad rush. Many people are deciding to walk and this has been going on since midday. Tomorrow there will certainly be a lot of "How long did it take you to get home last night?" tales.
The London Ambulance's official statement is that on top of the fatalities, 45 people have been seriously injured which includes horrific injuries like amputations, burn and fractured limbs. Over 300 are being treated for other injuries like shock and smoke inhalation.
The Docklands Light Railway is running again, but it is not stopping at interchanges with London Underground stations like Bank, Stratford and Canary Wharf.
[17.30 - Update from Annie]
I just heard Rudolph Giuliani on the radio who was New York's Mayor during September 11th who is in London at the moment and he was quite near Liverpool Street station during this morning's attack. He is obviously in total sympathy with London and is amazed with our "stoicism", a term that we've heard bandied around a lot today and it's great that people are dealing this all as calmly as they can.
Tony Blair is making another statement at Downing Street, now offering profound condolences to the family of the victims. "There will of course now be the most intense police and security action to catch the perpetrators....I would also pay tribute to the stoicism of the people of London....
"We know that the terrorists act in the name of Islam but we know that Muslims at home and abroad abhor this act as much as we do.......They are trying to use the slaughter of innocent people to frighten us and cower us out of doing the things we want to do. Of trying to stop us from doing the things we're entitled to do. They should not and they must not succeed. We will not be initimidated. We will not be changed. When they try to divide our people and weaken our resolve we will not be divided and our resolve will remain strong.....
"The purpose of terrorism is just that. It is to terrorise people and we will not be terrorised. This is a very sad day for British people but we will hold true to the British way of life"
Limited services are running on most mainline stations out of London including Waterloo.
[18.00 - Update from Annie]
News just come in advising people to stay away from Victoria station as a suspect package has been reported on a bus near the station.
Fatalities have now increased to 37 - this includes two confirmed deaths on the bus.
[19.33 - Update from Annie]
It's been a long and tiring day and I've only been blogging it and listening to radio and watching TV. Just spent the last hour and a half watching TV reports. Some really harrowing viewing including a guy who was on a motorbike behind the bus that was blown up - his mobile phone pictures were horrific.
There was also some mobile phone video footage of some people on one of the trains. I can't imagine how scary it must have been as obvisously all the power went off so you would have been in darkness for a while before emergency power came on. There's a lot of mobile videos shots on BBC London's site although it may take a while before you can view them.
Fortunately the bus driver at Tavistock Square is alive, but he would have seen the bomber get on the bus, and will be questioned by the police to see if he can recollect anything from the morning.
If you are still stuck in London bbc London's website seems to be the best source for travel updates. However from the TV pictures it looks as though a lot of people got out of London earlier in the day and the streets are eerily quiet.
There's not really much more for me to say today, so unless anything major is discovered, I'll close for the day.
Thanks for all the messages of support and keep making comments (there are over 100 below even though haloscan shows zero). London is standing firm and I'm sure we'll still see millions of people commuting tomorrow. We simply cannot let acts of terrorism grind us down.
Our thoughts are with all of the injured, the family and friends of the fatalities and everyone who has been caught up in this dreadful situation.
Listen to the radio or keep clicking on Google news for the latest. But the whole of the London Underground has been closed for the day. Completely unprecedented in my memory.