Explosions struck London's transport system Thursday, shutting down three underground train stations and blowing out the windows of a double-decker bus, exactly two weeks after four deadly suicide bombings.

One casualty was reported in the confusing series of incidents that bore similarities to the July 7 attacks, when four suicide bombers killed 52 people and themselves on three underground trains and a bus.

The bombs appeared to be smaller than the ones that exploded July 7, two weeks ago to the day, and appeared to detonate simultaneously, said Metropolitan Police Commission Ian Blair.

The Warren Street, Oval and Shepherd's Bush Underground stations were evacuated, and police were investigating a blast on a double-decker bus in east London.

Firefighters and police with bomb-sniffing dogs sealed off city blocks, evacuating restaurants and offices.

One man was escorted from Downing Street where Prime Minister Tony Blair was holding meetings. It was unclear whether the man was detained in connection with the explosions.

Prime Minister Blair appealed for calm, saying he hoped public transport services would return to normal quickly.

``We can't minimize incidents such as this,'' he said, flanked by Australian Prime Minister John Howard, who was on a visit to Britain.

``They're done to scare people, to frighten them and make them worried.''

Emergency crews found no traces of chemical agents at subway stations where explosions were reported.

Police were reportedly searching for a man ``with wires sticking out'' of his shirt at the University College Hospital near the Warren Street station.

Susie Sanders, who works at the hospital, said employees received a warning alerting them to be on lookout for an Asian man with wires protruding from his shirt.

Fidelna Fitzgerald said she saw about 30 armed police officers in the building. They shut the doors to the building and wouldn't let anyone out for hours.

It was ``panic basically ... I've never seen so many policemen in my life, with guns as well,'' she said.

Metropolitan Police Commissioner Ian Blair said: ``We know that we have four explosions or attempts at explosions and it is still pretty unclear as to what has happened.''

Witnesses said they heard reports of one or more exploding rucksacks.

Stagecoach, the company which operates the double-decker bus hit, 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 it had no reports of injuries.

``They were saying there was a bag left on the Underground,'' said Fethi Brandou, a 36-year-old gardener who saw people being evacuated.

One witness said he had been told by another subway passenger that a rucksack had exploded on a train.

``People were panicking. But very fortunately the train was only 15 seconds from the station,'' witness Ivan McCracken said.

McCracken said another passenger claimed he had seen a rucksack explode. The July 7 bombs were carried in rucksacks, police said.

McCracken said he smelled smoke and that people were panicking and coming into his carriage. He said he spoke to an Italian man who told him 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.''

Services on the Victoria, Northern and Hammersmith and City lines were suspended, London Underground said. ASSOCIATED PRESS