Here is a timeline for Thursday's London Underground incidents (all times local):
12:38 p.m.: Emergency services called to Oval Station, South London. Three ambulances sent to the scene.
12:45: Emergency services called to Warren Street Station, Central London. Five ambulances sent to the scene.
12:50: Emergency services called to Shepherd's Bush Station, West London.
1:15: Police confirm emergency services are at three stations. Stations sealed off.
1:19: London Fire Brigade say there are reports of smoke coming from Oval station, which crews are investigating.
1:30: First reports of an "incident" on a bush in Hackney, East London
1:30: Unconfirmed reports of nail bomb blast and possible gunshots at Warren Street Station. Other reports say the sound is detonators exploding, not firearms.
1:31: London Underground announces suspension of the Hammersmith and City, Victoria and Northern lines.
1:32: Scotland Yard confirms reports of an "incident" on a No 26 bus on Hackney Road and Columbia Road in Shoreditch. Street is sealed off.
1:45: London Underground confirms "Code Amber" on affected lines. Tube trains are stopped at the next station and the system is evacuated.
1:50: One injury reported at Warren Street.
1:50: Scotland Yard says emergency services are not treating the events as a "major incident, yet."
1:51: Stagecoach reports no injuries on Number 26 bus in Shoreditch. Bus driver reports windows on top floor have blown out.
2:00: Sky News reports witnesses of the bus incident saying a device exploded on the upper deck, blowing out the windows, but not injuring anyone.
2:04: Tony Blair delays photocall with John Howard, the Prime Minister of Australia, to investigate incidents.
2:10: Reports of armed police sealing off University College Hospital as three officers pursue a man inside.
2:10: London Underground confirm four lines suspended: Hammersmith and City, Victoria, Northern and Bakerloo.
2:14: Network Rail announces national rail services unaffected.
2:28: "We are dealing with an incident at University College Hospital and we have armed officers deployed there," says Scotland Yard. "We cannot go in any further detail at this stage."
2:31: Metropolitan Police Commissioner Sir Ian Blair tells Londoners "Stay where you are" in what he describes as a "very serious incident". Transport system will be shut down "for a short while" while police investigate.
2:31: Sir Ian describes incident at Shepherd's Bush as taking place "above ground".
2:36: London Underground reports Victoria Line has reopened either side of Warren Street.
2:45: Police report no trace of chemical agents found at Oval Station. Police in protective clothing are examining Warren Street Station.
3:03: Sky News reports staff at University College Hospital are warned to not to approach a tall black or Asian man, wearing a blue top with a hole in the back with wires protruding.
3:16: Ken Livingstone cancels his afternoon engagements.
3:22: British Transport Police report no trace of chemical agents found at Warren Street.
3:25: BBC reports a man arrested by two armed police outside the Ministry of Defence within yards of Downing Street in Whitehall. The area is sealed off as police confirm a "suspicious incident".
3:43: Tony Blair says he is unwilling to "minimise" the incidents, but says he hopes "things can get back to normal as soon as possible".
3:44: The Prime Minister reports there have been no casualties and says he will resume his schedule of meetings for the day.
3:50: Metropolitan Police Commissioner Sir Ian Blair says situation is "fully under control" and there has been one injury, but no deaths, in the four attempted attacks.
3:51: Sir Ian says attacks were "almost simultaneous" and that no traces of chemical agents have been found at any of the sites.
4:00: Incident at University College Hospital reportedly over.
4:10: Large area still sealed off around the number 26 bus in Shoreditch, East London. Reports say there could be explosives on the bus which may still be dangerous.
4:30: A police press conference will take place at 5:30 p.m.
5:45: Sir Ian Blair confirms there were four scenes - at Oval, Warren Street and Shepherd’s Bush Underground stations and on a bus in East London - where "attempts have been made to set off explosive devices". London Ambulance Service took no casualties to hospital. He added that some of the devices remained unexploded. Sir Ian refused to confirm suggestions that some of the devices were nail bombs.