But unlike the serial blasts that rocked the British capital's transport system on 7 July, killing 56 people and wounding hundreds of others, only one person was reported hurt on Thursday.
London city police chief Ian Blair called the latest explosions a "very serious incident".
"The intention must have been to kill," he said. He said casualties were light and the bombs appeared to be smaller than the ones that wreaked deadly havoc two weeks ago. Passengers on at least two of the London Underground trains told of would-be bombers fleeing after the small, near-simultaneous lunchtime explosions, which police said might have been detonators going off but failing to trigger a bomb.
Stagecoach, the company that operates London's double-decker buses, said the bus driver heard a bang and went upstairs, where he found the windows blown out. The company said the bus was structurally intact and it had no reports of injuries.
CCTV cameras on Hackney Road showed the bus immobilised at a stop with its indicator lights flashing. The area around the No 26 bus had been cordoned off and there was no traffic and no pedestrians.
In the video images, there appeared to be nobody on board or nearby and the streets appeared to have been cordoned off.
"The intention must have been to kill"
London police chief
London's Metropolitan Police later said the events were not on a par with the 7 July attacks, but had the potential to have been deadly, if not worse.
On 7 July, four young British Muslims detonated rucksack bombs in three packed trains and a bus at morning rush hour, killing more than 50 people as well as themselves.
Thursday's attackers either failed to wreak their intended havoc, or lacked the same lethal sophistication, according to experts.
The quick succession of multiple incidents and the initial confusion were reminiscent of that day.
British Transport Police said one person had been hurt at Warren Street underground station, but did not reveal how serious the injury was.
One witness said he had been told by another passenger that a rucksack had exploded on a train.
Prime Minister Tony Blair, who has rejected accusations that the invasion of Iraq has made Britain a target for Islamic hardliners, spoke at a press conference a few hours later, with visiting Australian Prime Minister John Howard at his side.
"We know why these things are done. They are done to scare people... We've got to react calmly," Blair said from Downing Street.
The incidents took place two
weeks after the 7 July attacks
Security analysts said the obvious carbon-copy attacks could have been masterminded either by the same group or by less sophisticated sympathisers.
"There is a resonance here," police chief Blair said, but he cautioned it would take time to tell who was to blame. He said some of the four "explosions or attempts at explosions" seemed not to have gone off properly.
"This may represent a significant breakthrough," he said. "There is obviously forensic material at these scenes which may be very helpful to us."
Police said Warren Street, Shepherd's Bush and the Oval underground stations had been evacuated. Emergency services personnel were called to the stations, police said.
Police cordoned off streets near Warren Street, and officers with sniffer dogs checked the area.
"I was in the carriage and we smelt smoke - it was like something was burning"
London Underground passenger
An eye witness said he smelled smoke and that people were panicking and coming into his carriage. He said he spoke to an Italian man who was comforting a woman after the evacuation.
"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," he said.
"The man then made an exclamation as if something had gone
wrong. At that point everyone rushed from the carriage."
Services on the Victoria, Northern and Hammersmith and City lines were suspended following reports of a number of incidents, London Underground said.
"I was in the carriage and we smelt smoke - it was like something was burning," said Losiane Mohellavi, 35, who was evacuated at Warren Street.
"Everyone was panicked and people were screaming. We had
to pull the alarm. I am still shaking," said Mohellavi, who was on the way to a job interview.
He told The Associated Press he did not see smoke, but smelled something similar to an electrical fire.
London Ambulance said it was called to the Oval station at 12.38pm and Warren Street at 12.45pm.
Hunt for perpetrators
All four sites remained sealed off by police as commuters struggled home on a disrupted public transport network.
Police sources said they were hunting several fugitives after the attacks and confirmed they had arrested two men - one near Downing Street - but could not connect them with the explosions.
An explosion took place on a bus
in east London
Armed officers later swarmed into a hospital near Warren Street amid media reports that they were looking for a man with wires protruding from the top he was wearing.
Friday's British newspapers focused on the fact that the perpetrators appeared to have got away.