LONDON - Up to four bombs hit London's transport system on Thursday, exactly two weeks after more than 50 people were killed in blasts on underground railway trains and a bus.
London police chief Ian Blair said there were four explosions or attempted explosions in what he called "a very serious incident".
He said casuatlies were light and the bombs appeared to be smaller than the ones that wreaked deadly havoc two weeks ago.
Witnesses said they had seen what could have been a would-be bomber running away after dropping a rucksack on one of the trains.
"We all got off on the platform and the guy just ran and started running up the escalator," one witness who gave her name as Andrea told the BBC.
"Everyone was screaming for someone to stop him. He ran past me... and he ran out of the station. In fact he left a bag on the train," she said.
Passengers were evacuated off a bus in Hackney, east London, and police cordoned off streets nearby. The bus company said a blast blew out the windows of the bus but a police officer on the scene said there were no signs of damage.
A police officer told Reuters: "The bus driver heard a bang at the back of the bus. He thought it was probably a vehicle that had hit him.
"He stopped at a nearby bus stop and saw a suspect package at the back of the bus."
Bomb attacks on three undergound trains and a bus in London killed 56 people, including the four bombers, on July 7.
Armed police entered a hospital near one of the incidents at Warren Street in central London and immediately cordoned off the building.
"Three armed policemen just ran into the major incident area entrance of University College Hospital," a Reuters correspondent said.
The Fire Brigade said Warren Street underground station was evacuated after reports smoke was seen coming out of a train.
A television reporter at Oval underground station said police had cordoned off the area and brought in sniffer dogs.
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