London blasts cause chaos on Tube
Police in protective clothing were deployed at the bus site
The Tube has been plunged into chaos and several stations evacuated after minor blasts on three trains and a bus.
Met Police chief Sir Ian Blair said three Tube lines were suspended but it was time London returned to normal.
The minor explosions - just two weeks after blasts killed 56 - involved detonators only, a BBC reporter said. There was only one injury.
Police sources say a number of fugitives are being sought. Two people were arrested in Whitehall.
Eyewitnesses heard bangs and saw abandoned rucksacks at the sites of the incidents at Warren Street, Shepherd's Bush Hammersmith and City line and Oval tube stations as well as the number 26 bus.
At Warren Street and Oval a man was seen running away from the scene.
A blast was also reported on a Number 26 bus in Bethnal Green.
There were no injuries and the bus suffered no structural damage.
Large areas around all four sites were cordoned off.
One person was injured at Warren Street. There were reports the injured person may have been holding a rucksack containing the detonator.
Sir Ian appealed for witnesses with mobile phone pictures of any of the incidents to send them to www.police.uk
And he said it was "time to get London moving" again.
Prime Minister Tony Blair said: "We can't minimise incidents such as this because they obviously have been serious in the four different places as we know.
"I think all I'd like to say is this that we know why these things are done, they're done to scare people and to frighten them, to make them anxious and worried."
The BBC's Andrew Winstanley said devices had been found but appeared to have been dummies, containing no explosives.
Police said armed officers were deployed to University College Hospital after an incident. A large area was cordoned off, but the incident is now over.
There were reports a memo had been circulated to staff to look out for a 6ft 2in black or Asian man with wires sticking out of his top.
The hospital has not received any casualties or been alerted to casualties.
A man was arrested near Downing Street by armed police and led away down Whitehall.
The whole of the Northern Line has been suspended, along with the Victoria Line, the Hammersmith and City line, Piccadilly and the Bakerloo line.
Police have set up cordons round the stations
A number of other stations were closed including Great Portland Street, Westminster, Waterloo, St Paul's and Oxford Circus tube stations, as well as Waterloo tube station and King's Cross Thameslink.
Tony Blair cancelled events in the afternoon and attended a meeting of the Cobra committee along with Sir Ian. Whitehall was briefly closed down.
London Underground went to an amber alert with trains taken to the next station and evacuated.
An eyewitness at Oval station said there had been a small bang, and a man had then run off when the Tube reached the station.
A spokesman for Stagecoach said the driver of the number 26 bus travelling through Shoreditch had heard a bang on upper deck, gone upstairs and seen the windows were blown out.
The bus driver was very shaken but said to be fine.
At Shepherd's Bush Hammersmith and City line station, police told reporters that a man had threatened to blow himself up and then ran off.
Sosiane Mohellavi, 35, was travelling from Oxford Circus to Walthamstow when she was evacuated from a train at Warren Street.
"I was in the carriage and we smelt smoke - it was like something was burning. "Everyone was panicked and people were screaming. We had to pull the alarm. I am still shaking."