Warren Street, Shepherd's Bush and Oval stations were all evacuated after the blasts. A small explosion then hit a No. 26 bus at Hackney at 1.30pm - when at least one of the upstairs windows on the bus was blown out.
Sir Ian Blair, the Metropolitan police commissioner, comparing today's incidents to the 7 July attacks, said: "These are smaller devices, some of them have not gone off properly. We may have recovered forensic material. It may be important to our investigation."
He said that the situations were under control and nothing indicated any form of chemical attack. He added: "Now is the time to get London moving again."
He said later that he did not want to say too much, adding: "At the end of this process we want to have some trials."
He did say, however, that the forensic material available may represent a significant breakthrough. And he asked Londoners who had photos and images from the scenes to send them to the police to help with the investigation.
"The intention of the terrorists has not been fulfilled," he said.
The Victoria, Hammersmith & City and Northern lines remain closed - but other lines carried on as they had been in the wake of the 7 July bombings. Overground trains are running normally.
The Prime Minister Tony Blair said: "We know why these things are done - to scare people and to make them anxious. Fortunately there appear to be no casualties and I think we have just got to react calmly."
Police launched an operation at University College Hospital, which is near Warren Street station. Sky News reported that staff were asked to look out for a 6ft 2in male, possibily of Asian appearance, wearing a blue top with wires coming from the back of it. The hospital is near Warren Street station. Police later said that operation had been "stood down" - but large numbers of armed officers soon returned.
Two people were arrested this afternoon - one in Downing Street and one in Great Scotland Yard in Whitehall - since the incidents this afternoon. However, police sources said it was unclear at this stage whether the two had any links to the explosions.
Meanwhile eye-witnesses gave accounts of what happened on the Tube.
Sosiane Mohellavi, 35, was travelling from Oxford Circus to Walthamstow when she was evacuated from the 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. We pulled into Warren Street and were evacuated. It was horrible."
Victoria Line passenger Ivan McCracken said a traveller's rucksack had exploded on the Tube outside Warren Street station.
He told Sky News: "I was in a middle carriage and the train was not far short of Warren Street station when suddenly the door between my carriage and the next one burst open and dozens of people started rushing through. Some were falling, there was mass panic.
"It was difficult to get the story from any of them what had happened but when I got to ground level there was an Italian young man comforting an Italian girl who told me he had seen what had happened.
"He said that a man was carrying a rucksack and the rucksack suddenly exploded. It was a minor explosion but enough to blow open the rucksack. The man then made an exclamation as if something had gone wrong. At that point everyone rushed from the carriage."
At Oval station there were reports of a man dumping a rucksack in a carriage then fleeing as the doors closed. A witness told Sky News he heard a sound "like champagne popping" then passengers erupting in panic.
"As far as I know from what a lady at the top of the escalator was saying, someone came into the carriage, dumped the bag and ran out. Some people tried to stop him but he ran out."
He said: "I was in the carriage next to the one where the bag was. All of a sudden there was a popping, it sounded like champagne popping. I didn't think anything of it at the time but then I heard a lot of shouting from the next door carriage.
"People started saying, 'Smoke, smoke'. One of the train guys came through and said 'Get off the train, we're evacuating, everyone out'. As we were walking past the carriage we could see the bag sitting on the chair. It was a big, black rucksack, like the backpack-type ones that you get."
The Government's civil contingencies committee - known as Cobra - met this afternoon in response to the latest incidents, Downing Street confirmed.
It is two weeks to the day since the London bombings, when three suicide bombers detonated bombs on Tube trains in the capital and a fourth blew himself up on a bus.