NEW LONDON TERROR ALERT

Bombers have again targeted London's transport system - with up to four explosions reported. Metropolitan Police Commissioner Sir Ian Blair said the bombs appeared to be smaller than used in the attacks two weeks ago but advised people to "stay where you are". Passengers reported one blast at Warren Street station off Tottenham Court Road in central London. There has also been an incident on a bus in Hackney, East London. Stations at Warren Street, Oval and Shepherd's Bush have been closed.Scotland Yard have confirmed there is an incident involving armed police officers at University College Hospital in Bloomsbury close to Warren Street station. A man was also arrested by armed police at the gates of Downing Street. An internal memo to staff at the hospital has warned them to be on the lookout for a black male, possibly of Asian origin, about 6ft 2ins tall, wearing a blue top with wires protruding from the rear of the top. Police officers in chemical protection suits have been seen appearing to enter Warren Street, where British Transport Police say one person has been injured. A initial chemical search at Oval station has proved negative. Sir Ian told reporters at Scotland Yard the incidents were "serious". There appeared to have been a series of explosions or attempted explosions. He urged Londoners to "stay where you are and go about your normal business" for the time being. Tony Blair said that although the incidents had to be treated as seriously, there had been no reports of casualties. "We know why these things are done - they are done to scare people," he told a news conference. He said police were working to return London to normal as quickly as possible. Services on the Victoria, Northern and Hammersmith & City and Bakerloo Lines have been disrupted. Emergency services started receiving calls just after 12.30pm. At Warren Street there were reports of shots and a nail bomb explosion. Sky's Crime Correspondent Martin Brunt said police believe this may have been the sound of detonators going off.Victoria Line train passenger Ivan McCracken told Sky News he spoke to an Italian man who witnessed an explosion just after the train arrived at the platform. "He told me he had seen a man carrying a rucksack which suddenly exploded. It was a minor explosion but enough to blow open his rucksack. Everyone rushed from the carriage. People evacuated very quickly. There was no panic. "I didn't see anyone injured but there was shock and fright." One witness told Sky News that passengers tried to prevent a man with a rucksack running away but they failed. At Oval station there were reports of a man dumping a rucksack in a carriage then fleeing as the doors closed. Police cordoned off the streets around Warren Street station. Scotland Yard said emergency services responded to an "incident" on a Number 26 bus in Hackney Road, on a junction near Colombia Road, east London. Bus operator Stagecoach said the driver heard a bang at around 1.30pm. The bus had left Waterloo and was in Shoreditch when the incident happened. "The driver heard a bang which appeared to come from the upper deck. When he went upstairs to investigate, the windows on the upper deck were blown out. "The bus is structurally intact and we don't have any reports of injuries," said a spokesman. Network Rail say all mainline train services are running from London stations. Police are appealing for anyone with mobile phone images, pictures or video from any of the four sites to send in their images via a website www.police.uk. It is two weeks to the day since bombers attacked three Tube trains and a bus in central London.