A minor blast took place on a train approaching Warren Street
One of the explosions took place on a Victoria line train at Warren Street tube station.
A passenger said he was reading a book when he suddenly smelt something burning "like wiring or tyres", which got more intense.
Other passengers reported seeing a man with a rucksack which was blown open by a small explosion. Many dropped their belongings and tried to escape.
On Thursday evening, the area around the station was still cordoned off.
"I was on the Warren Street Tube when the driver made an unclear announcement - I heard the word 'emergency'," passenger Kim Howey told the BBC.
"When the doors opened we heard panicked people towards the front of the Tube screaming and running along the platform towards the back, then running through the passageway to the southbound platform and up the escalator," he added.
Jimmy Connor, 32, from Sheffield, was among those who left his bag on the train.
"I thought I was going to die, everyone else thought the same," he told PA news agency.
Others reported there had been relative calm on the train, and that some of the passengers had remained on the platform waiting to hear if the emergency was over.
Passenger Ivan McCracken told Sky News he met an Italian couple who said they had witnessed the blast.
They said a rucksack carried by a man was torn apart by a minor explosion. The man then "made an exclamation as if something had gone wrong," Mr McCracken quoted the couple as telling him.
Police later evacuated pubs and offices near Warren Street tube station, and sniffer dogs were deployed to comb the pavement.
A further alert was later sparked by reports that the bomber might have escaped and sought refuge inside the nearby University College Hospital.
Armed police went into the hospital and two people were reportedly held at gunpoint and questioned before eventually being released.
A police search went on for several hours, before hospital staff were eventually allowed to leave the building.
A large area near the hospital was still cordoned off in the evening.
The eastbound section of the Euston Road reopened around the Warren St area after 2100.
People living above the Tube station who were not in their homes at the time of the incident would be spending the night at Camden Town Hall at the request of the police.
Residents who live above the station were not allowed to return home
"We can't go back to the flats because although police say there is no danger, if you walk down the street you might disturb evidence," Martino Carpella, Secretary of the Residents Association for Warren Court said.
Eileen Moreland, 91, was among those who could not return home.
"I'm feeling a bit shaky because I haven't been very well and I find it difficult to walk," she said, after having spent half the day in the street.
Police have set up witness reception points at all sites of Thursday's blasts.
They are urging witnesses of the Warren Street incident to visit the centre at the British Transport Police Central headquarters, 16-24 Whitfield St W1 until 2300 on Thursday, or from 0730 until 2300 on Friday.