LONDON - Two weeks after suicide attacks on subway stations and a bus, police reported explosions at three subway stations and on a double-decker bus today.

Only one person was reported wounded, but the explosions during the lunch hour caused major disruption in the city and were hauntingly similar to the July 7 bombings in which 52 people and four suicide attackers were killed.

The London police commissioner confirmed that four explosions took place in what he described as "serious incidents."

"We've had four explosions- four attempts at explosions," Metropolitan Police Commissioner Ian Blair said outside police headquarters at Scotland Yard.

"At the moment the casualty numbers appear to be very low ... the bombs appear to be smaller" than the July 7 blasts.

Sky News TV reported that police said no chemical agents were involved in the explosions.

Police also said an armed police unit had entered University College hospital. Press Association, the British news agency, said they arrived shortly after an injured person was carried in.

Police in chemical protection suits were seen preparing to enter the Warren Street Underground station. Unspecified incidents also were reported at the Shepherds Bush and Oval stations.

Stagecoach, the company that 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 reports of injuries.

Closed-circuit TV cameras on Hackney Road showed the No. 26 bus immobilized at a stop with its indicator lights flashing. The area around the bus had been cordoned off.

Prime Minister Tony Blair canceled his afternoon appointments as the developments unfolded.

The incidents were similar to the blasts two weeks ago, which involved explosions at three Underground stations simultaneously - quickly followed by a blast on a bus. Those bombings, during the morning rush hour, also occurred in the center of London, hitting the Underground railway from various directions.

Today's incidents, however, were more geographically spread out.

London Ambulance said it was called to the Oval station at 12:38 p.m. and Warren Street at 12:45 p.m. The July 7 attacks began at 8:51 a.m.

British Transport Police said one person was hurt at Warren Street, but they did not know how serious the injury was.

BBC TV reporter Keith Doyle near Oval station said a police officer told him there had been an incident, although it was not an explosion. He said officers had cordoned off a wide area around the station.

"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 claimed he had seen a backpack explode. The bombs that killed 56 people on board three underground trains and a bus in London on July 7 were carried in rucksacks, police said.

Police cordoned off streets near Warren Street, and officers with bomb-detecting dogs checked the area.

McCracken said he smelled smoke and that people were panicking and coming into his carriage.

McCracken 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."

Services on the Victoria and Northern 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," Mohellavi said.