Witnesses describe smoke and panic in a jittery city

Up to four bombs hit London's transport system early this morning, exactly two weeks after more than 50 people were killed in blasts on Underground railway trains and a bus.

Warren St, Shepherds Bush and Oval stations were evacuated, as was a bus in Hackney, and at least four Underground lines were shut.

Witnesses said they saw what could have been a would-be bomber running away after dropping a rucksack on a train at Warren St.

"We all got off on the platform and the guy just ran and started running up the escalator," one 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."

Sky News TV reported that police at University College Hospital, which is near Warren St station, were searching for a tall man wearing a blue shirt with wires protruding from the back.

A short time later police sealed off the bottom end of Whitehall and several armed officers rushed a man standing directly opposite the Ministry of Defence building, across the road from the gates of Downing St.

The man was forced to lie down for two minutes while police surrounded him. He was handcuffed and led away at gunpoint.

London police chief Ian Blair said there were four explosions or attempted explosions in what he called "a very serious incident".

However, Sir Ian said casualties were light and the bombs appeared to be smaller than the ones that wreaked deadly havoc two weeks ago.

He said there were no chemicals.

A number of reports suggested that the explosions were caused by detonators rather than bombs.

By 3am New Zealand time the underground system was working again with the exception of the Hammersmith, City, Northern and Victoria lines.

WARREN ST

There was panic at the Warren St tube when passengers heard an explosion as a train was pulling into the station about 12.35am New Zealand time.

British Transport police reported one injury from the blast.

Eyewitnesses said they heard what sounded like a small explosion, then people moved through the doors between carriages to the back of the train.

One woman was sitting in a quiet carriage when people started moving through the very narrow doors. She said she could smell something like burning tyres.

As soon as the doors opened, people started shouting and screaming, clambering over each other trying to get out of the station.

Caroline Russell told BBC World she was in a carriage at the other end of the train heading into the Warren St station when she smelled a "nasty rubbery burning smell".

She said a boy who was at the other end of the train told her a rucksack carried by an Englishman exploded, "but not a big explosion, not lots of smoke".

The passengers went through the emergency door of the carriage and rushed out when the train pulled into the station.

Another eyewitness on the train coming into the Warren St station said there was a burning rubber smell, then someone pulled the emergency alarm.

"The train pulled in at the station ... some people rushed off screaming and panicking," followed by an announcement to evacuate.

The Fire Brigade said Warren St underground station was evacuated after reports smoke was seen coming out of a train.

"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 had claimed he had seen a rucksack explode.

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

OVAL STATION

Pete Elliott was on a train coming into the Oval station, when smoke suddenly started coming out from a carriage.

"A couple of guys starting shouting for everyone to leave as quickly as possible ... it was all pretty frightening."

A television reporter at the station said police had cordoned off the area and brought in sniffer dogs.

HACKNEY

An explosion blew out the windows of the a bus in the Hackney area of London.

"The driver heard a bang he believed came from the upper desk of the bus. The windows were blown out.

"There are no reports of any injuries," said a spokesman for the bus company Stagecoach.

The No 26 bus was travelling from Waterloo to Hackney in the east of city.

The explosion happened in the Shoreditch area.

The spokesman said the structure of the bus was intact but some windows were blown out.

Passengers were evacuated from the bus and police cordoned off streets nearby.

Fox News reported police fears that there were explosives in a rucksack on the upper deck.

SHEPHERDS BUSH

The BBC reported a lot of police activity at Shepherds Bush including a dog unit as well as numerous officers but there were no ambulances suggesting no injuries.