News of incidents at Warren St, Oval and Shepherd's Bush, plus the no 26 bus in Hackney.
No injuries reported but the tube network is down for the moment.
At Warren Street there were reports of a nail bomb explosion. Sky's Crime Correspondent Martin Brunt said police believe this may have been the sound of detonators going off.
Victoria Line train passenger Ivan McCracken told Sky News he spoke to an Italian man who witnessed an explosion just after the train arrived at the platform.
"He told me he had seen a man carrying a rucksack which suddenly exploded. It was a minor explosion but enough to blow open his rucksack. Everyone rushed from the carriage. People evacuated very quickly. There was no panic."
From the Guardian News Blog:
A London Ambulance spokesman has told PA they were called to Oval at 12.38.pm and Warren Street at 12.45pm. There are no details of the alert at Shepherd's Bush yet.
The BBC are now saying that the explosions were 'dummy blasts' consisting only of detonators. The Guardian are saying that "there was an attempted suicide bombing at Warren Street, but the bomb didn't go off properly. Some windows were blown out the bus at Hackney, it is reported. Scotland Yard says it is not treating this as a major incident, yet."
Eyewitness report from Sky News:
Someone dumped a large, black holdall or rucksack on the train and then ran off. Other passengers tried to stop him running away, but he escaped. Something in the bag went off, making a sound "like a Champagne cork popping".
One injury has been reported at Warren St but it may have arisen from the rush to evacuate the train.
14:30:Warren St witness on BBC News 24 said there was a terrible smell of burning rubber but no smoke. Then an alarm rang and the driver announced they were coming into Warren St. The doors opened but no one in her carriage moved until people from the front of the train came running past the platform. Once above ground she was told by a person who was in a carriage further along that a British guy's backpack exploded but it was more like a firework than a real explosion.
14:40 Shepherd's Bush Green has been closed off between Holland Park and the Shepherd's Bush Station due to a suspect package.
14:58: Sky reports that Victoria line open either side of Warren Street.
15:00: Blair is due to give a statement around 3:15.
15:20: Scotland Yard have confirmed there is an incident involving armed police officers at University College Hospital in Bloomsbury, where one of the bombers may have escaped into.
15:26: Reports of an arrest at Whitehall
15:34: Right now it seems that the Hammersmith & City, Victoria and Northern lines are down.
15:37: The BBC are showing pictures of armed police making an arrest outside Downing Street.
16:25 Our servers are running very slowly right now so it's proving difficult to update the front page. Bear with us and hopefully it will get sorted. Things seem to be calming down and we're expecting the tubes lines to reopen pretty quickly.
16:45 Summary: All the explosions involved detonators only and there has been only one injury reported. Two people were arrested in Whitehall and "a number of other fugitives" are being sought.
The bus in Bethnal green suffered no structural damage (although the windows were blown out) and no one was hurt.
The incident at University College Hospital is now over.
There's conflicting reports about the Tube but we know the whole of the Northern Line has been suspended, along with the Victoria Line, the Hammersmith and City line, and the Piccadilly and the Bakerloo lines.
Oxford Street station is currently closed "and will be for some time"
Other closed stations are Great Portland Street, Westminster, Waterloo, St Paul's. Waterloo tube station and King's Cross Thameslink are closed for the moment.