Blasts at Underground stations and on a bus caused panic in London last night just two weeks after fatal attacks that claimed the lives of 56 people and wounded hundreds.

At least one person was injured after a suspected nail bomb detonated near Warren Street Underground station, in the British capital.

Another explosion rocked a bus in the Hackney area of east London, blowing out the windows of the top deck.

‘The driver heard a bang he believed came from the upper desk of the bus,’ a Stagecoach spokesman said, adding that the windows were blown out. There are no reports of any injuries.

Scotland Yard officials said Warren St, Shepherd’s Bush and Oval stations had been evacuated and emergency workers were called to the stations.

London’s police chief Ian Blair said the explosions were clearly ‘a very serious incident’.

He said there were four explosions or attempted explosions but they appeared to be smaller than the ones that rocked the city two weeks ago.

British Prime Minister Tony Blair interrupted lunch with Australian PM John Howard to be briefed on the emergency.

Eyewitness passengers aboard a Tube train at Warren St said they saw a man dump a sack inside a carriage seconds before it exploded.

‘Someone came into the carriage, dropped the bag and ran out, people tried to stop him but he made off,’ said a passenger. ‘It sounded like a champagne corking. I feel a bit shaken.’

A passenger at Warren St said about 30 passengers were inside the carriage when the blast occurred. ‘I was in the next carriage and I could detect smoke,’ he said.

Another witness said: ‘We all got off on the platform and the guy just ran and started running up the escalator. 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.’

Reports said armed police entered a hospital near Warren St and immediately cordoned off the building.

‘Three armed policemen just ran into the major incident area entrance of University College Hospital,’ Reuters correspondent Gerard Wynn reported from the scene. ‘The hospital has now been sealed off.’