Counter-terrorism officers have staged raids in London to arrest six people over an alleged plot to potentially attack targets in Britain.
One person was shot with a stun gun and Scotland Yard said armed officers had been deployed during the arrests, which is an unusual decision and indicative of the risk counter-terrorism chiefs thought officers faced in detaining the suspects.
Five men and one woman were arrested in what police said was an intelligence-led operation that is believed to have gone on for several weeks.
The six were arrested on suspicion of the commission, preparation or instigation of acts of terrorism. Officials believe they are dealing with a plot inspired by al-Qaida ideology.
Police said the arrests were not linked to any threat to the Olympic Games, but officials say they will intervene quicker than usual in disrupting alleged plots ahead of the Games, which counter-terrorism officials fear is a major target.
The six arrested were taken to a south-east London police station where they will be questioned.
In another unusual feature, a 29-year-old man was arrested in the street in west London.
Usually police prefer to arrest suspects while they are at home.
The Metropolitan police said the arrests included a 21-year-old man and a 30-year-old woman who were arrested at separate residential addresses in west London.
Three other men, aged 18, 24, and 26, were arrested in east London at residential addresses.
Police said: "Searches under the Terrorism Act 2000 are being carried out at eight residential premises in east, west and north London and one business premises in east London.
"Public safety remains our overriding concern.
"Today's operation in London is not linked to the Olympic and Paralympic Games."
They added: "The men and woman were all arrested on suspicion of the commission, preparation or instigation of acts of terrorism under the Terrorism Act 2000."
It is understood MI5 played an active role in the intelligence-led operation. Investigative sources suggested there was no imminent threat but that the police believed there was enough evidence to act.
MI5 and the police have said repeatedly that they are monitoring a number of groups that could pose a threat to national security, and they have been focusing on "nipping plots in the bud" rather than letting them mature.
Though the arrests were not linked to the Olympic Games, Scotland Yard and MI5 are on a high state of alert with less than a month to go before the event. Police may make more arrests in the coming weeks as officers become more risk-averse.