London's Tube network has been plunged into chaos with stations cleared after minor blasts on two trains and a bus.
Emergency services in protective clothing were deployed at the bus site
Met Police chief Sir Ian Blair said three Tube lines were suspended but it was time London returned to normal.
The minor explosions - two weeks after blasts killed 56 - involved detonators only, a BBC reporter said. There was one injury.
Police sources say the blasts may have been near simultaneous and that they are being linked with the 7 July bombs.
They say a number of fugitives are being sought. Two people have been arrested in Whitehall.
Detectives are recovering a lot of evidence from the sites, and believe the latest events may either be a repetition of the 7 July attacks or may help with a breakthrough in the investigation.
Eyewitnesses heard bangs and saw abandoned rucksacks at the sites of the incidents at Warren Street and Oval tube stations as well as the number 26 bus in Bethnal Green.
There was an attempt to cause an explosion at Shepherd's Bush Hammersmith and City line, police said.
Police told reporters that a man had threatened to blow himself up and then ran off.
At Warren Street and Oval a man was seen running away from the scene.
On the bus, there were no injuries and the bus suffered no structural damage.
Large areas around all four sites were cordoned off. Tests for chemical, biological and radiological weapons proved negative.
One person was injured at Warren Street. There were reports the injured person may have been holding a rucksack containing the detonator.
In other developments:-
- The whole of the Northern Line has been suspended, along with the Hammersmith and City line.
- University College Hospital was cordoned off twice, with armed police entering the building. There were reports they were searching for a suspect from the Warren Street incident.
- Foreign Secretary Jack Straw, Sir Ian Blair, Lord Chancellor Lord Falconer, Attorney General Lord Goldsmith, Immigration Minister Des Browne and Transport Secretary Alistair Darling attended Downing Street for a meeting with the prime minister.
- There were also alerts at Wood Lane in Shepherd's Bush, around St Paul's and, outside London, at St Albans station.
- A number of other stations were closed during the alert, 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.
- Sir Ian appealed for witnesses with mobile phone pictures of any of the incidents to visit the www.police.uk website.
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."
Police have set up cordons round the stations
Sofiane Mohellebi, 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."