1620: Two people are detained, police say. A police source says officers are looking for an unknown number of fugitives.
1610: No traces of chemical agents found at any of the incident sites, police say.
1559: Incident at University College Hospital is "stood down", Scotland Yard announces.
1547: Prime Minister Tony Blair says incidents such as Thursday's are designed to frighten people, adding: "We've just got to react calmly and continue with our business - as much as possible - as normal."
1545: Metropolitan Police Commissioner Sir Ian Blair says the situation is "fully under control". He says there is no indication of chemical or other attack, and that there has been only one casualty - not a fatality.
1525: Armed police arrest a man outside the gates of Prime Minister Tony Blair's 10 Downing Street offices.
1522: Police say no trace of chemical agents is found at Warren Street station following tests.
1503: Police appeal for anyone with photos or mobile phone images from the incident scenes to send them to www.police.uk.
1443: Police ask people to remain at work or at home.
1430: UK Government's emergency-response team Cobra meets at Downing Street, with Prime Minister Tony Blair, Foreign Secretary Jack Straw and Police Commissioner Sir Ian Blair attending.
1428: Metropolitan Police Commissioner Sir Ian Blair says Thursday's incidents are "clearly very serious" but that the explosions were smaller than those which killed at least 56 people two weeks ago. He says the transport system will close down "for a short while".
1426: Police in protective gear deployed to Warren Street station.
1413: Prime Minister Tony Blair postpones a visit to a school in east London and a photocall with visiting Prime Minister John Howard of Australia, a spokesman says.
1411: Police cordon off a large area around University College Hospital, which is near Warren Street station.
1401: A White House spokesman says the US is "monitoring the situation closely".
1354: Police say they are not treating the evacuations as a "major incident yet".
1348: Westminster Underground station is evacuated.
1342: Police respond to reports of an explosion on a Number 26 bus in Hackney Road in east London. There are no reports of injuries.
1340: Ken Livingstone, the mayor of London, cancels an engagement because of the incident on the Underground, a spokeswoman says.
1325: London Underground announces services suspended on three lines, Hammersmith & City, Victoria and Northern.
1321: Fire service says smoke seen coming from Warren Street Tube station.
1320: The pound sterling falls against other currencies as word of the incidents hits the markets. The index of leading shares on London's stock exchange also falls.
1311: Shepherd's Bush station on the Hammersmith and City line is evacuated.
1245: Ambulance services are called to the Warren Street station on the Northern and Victoria lines. The station is evacuated.
1238: Ambulance services are called to the Oval Underground station on the Northern Line. The station is evacuated.