London's police commissioner says the situation is "under control" after four small explosions Thursday aboard three Underground trains and a bus.
One injury was reported in a chilling but less deadly replay of the suicide bombings that killed 56 people two weeks ago.
"Clearly the intention must have been to kill," Ian Blair told a news conference. "You don't do this with any other intention."
He also said it was not clear if the two sets of attacks were connected.
Panicked and screaming commuters fled the three affected Underground stations, sometimes leaving behind their shoes. Firefighters and police with bomb-sniffing dogs sealed off nearby city blocks and evacuated rows of restaurants, pubs and offices.
"We can't minimize incidents such as this," he said at a joint news conference with the Australian prime minister at No. 10 Downing St. "They're done to scare people, to frighten them and make them worried."
He held an emergency Cabinet meeting afterward but said no decisions "of a policy nature" were made.
Ian Blair called the blasts "a very serious incident."
"We know that we have four explosions or attempts of explosions, and it is still pretty unclear as to what has happened," he said outside Scotland Yard.
"At the moment the casualty numbers appear to be very low ... the bombs appear to be smaller" than those detonated July 7, he said.
An armed police unit entered University College hospital shortly after an injured person was carried in, Britain's Press Association reported.
Sky News TV reported that police were searching for a man with a blue shirt with wires protruding. Officers asked employees to look for a black or Asian male about 6-foot-2.
The attacks, which targeted trains near the Warren Street, Oval and Shepherd's Bush stations, did not shut down the subway system, only three of its lines. The double-decker bus had its windows blown out on Hackney Road in east London.
Witness Jimmy Conners, who was aboard the train at Warren Street, told CBS News there was "the smell of burning wire… we thought we were all going to die… everyone was just panicking and heading for the doors."
Conners said it was "definitely the smell of death. Everyone on that train thought they were going to die."
CBS News Correspondent Richard Roth reports another eyewitness described hearing "explosions that sounded like champagne corks popping."
Police in chemical protection suits were at the Warren Street station, but no chemical agents were found.
Stagecoach, the company which operates the stricken bus, said the driver heard a bang and went upstairs, where he found the windows blown out. The company said the bus was structurally intact and there were no injuries.
The incidents paralleled the July 7 blasts, which involved explosions at three Underground stations simultaneously starting at 8:51 a.m., followed quickly by a bomb going off on a bus. Those bombings, during the morning rush hour, also occurred in the center of London, hitting the Underground from various directions.
Thursday's incidents, however, were more spread out.
Emergency teams were sent to all three stations after the incidents, which began at 12:38 p.m.
"People were panicking. But very fortunately the train was only 15 seconds from the station," witness Ivan McCracken told Sky news.
McCracken said another passenger at Warren Street told him he saw a backpack explode. The July 7 bombs were carried in backpacks, police said.
McCracken said he smelled smoke, and 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," McCracken said. "The man then made an exclamation as if something had gone wrong. At that point everyone rushed from the carriage."
The U.S. Embassy was closed to visitors about two hours after the blasts as a precaution, but embassy staff continued working, said spokeswoman Susan Domowitz.
"This would be the first time we've seen follow-up attacks following any major attack, anyplace in the world,"
Livingstone said the possible implications of the new incidents are frightening. "This sends a very powerful message, and what they're saying is, 'No matter how great your network of informants and cameras and so on, you still can't stop us. You can arrest us. We can have people that are killed in these attacks, but we have other people that are willing to carry out further attacks. And so we're going to be there, and we're going to be there for a long time.'"
The explosions came as Pakistani intelligence officials said authorities are seeking the former aide of a radical cleric in Britain in connection with the July 7 bombings.
The officials said British investigators asked Pakistani authorities to search for Haroon Rashid Aswat, who reportedly had been in close contact with the suicide bombers just before the July 7 attacks.