Four British men are being questioned over an alleged terror plot after they were arrested in a series of raids by armed police officers across London.
The men - arrested at about 19:10 BST - are being held on suspicion of the commission, preparation or instigation of acts of terrorism.
Searches are continuing at six addresses and on two vehicles.
Whitehall officials said the alleged Jihadist plot was "serious" and intended to use firearms in the UK.
But they said it did not appear to be on anything like the scale of earlier major plots, such as the airline liquid bomb plot or the Birmingham rucksack bomb plot, which resulted in convictions.
The men were taken to a south London police station.
Two men, both aged 25, were arrested in a car in Mansell Street, Whitechapel, east London, after police fired "Hatton rounds" - ammunition designed to blow out tyres and blow open doors.
One was a British national of Turkish origin and the second was a British national of Algerian origin, police said.
A 28-year-old British national of Azerbaijani origin was arrested at premises in Westbourne Grove, Bayswater, west London, and the fourth man, a British national of Pakistani origin, aged 29, was arrested in Peckham Hill Street, Peckham, south-east London.
BBC security correspondent Frank Gardner said the decision to make arrests was taken on Sunday by a joint committee of senior officers from MI5 and the Met Police's Counter Terrorism Command.
As always, they faced a dilemma over whether to hold off longer to gather more potential evidence, while running the risk that the public might be in danger, our correspondent said.
The alleged plot had been monitored for some time, he added.
He also said the four men must either be charged or released within 48 hours unless a magistrate grants a warrant to extend their detention for up to two weeks.
George Paul, who saw the Bayswater arrest, said it had been "very dramatic, like the movies".
"What I saw was three policemen... holding an individual down while he was shouting 'please don't break my arms' or something of that ilk," he told BBC News.
End Quote Alistair Gillies
It looked like there was a great deal of planning and a great deal of resources had been put on to tackling this situation"
"They were basically trying to restrain him with another few officers ushering people away from the area."
And Ramin Massodi, who watched the arrest from inside the Persian restaurant where he works, said: "I heard shouting; then I looked outside and saw four cars and they grabbed him."
He said police officers pushed the suspect up against the glass of the restaurant.
Alistair Gillies, 38, who saw the same arrest, told BBC News he estimated there had been about two dozen officers involved.
"The majority of them appeared to be plain clothes but they had on baseball caps with the checkerboard and police marking. Apart from that, they were dressed in plain clothes," he said.
"It looked like there was a great deal of planning and a great deal of resources had been put on to tackling this situation."
Meanwhile, City of London council member John Fletcher, whose apartment backs on to Whitechapel's Mansell Street, said he initially heard two loud bangs which he believed to be fireworks.
He said it was only when he went to investigate that he realised the seriousness of the situation.
"[A policeman] said: 'You'll hear all about it in the papers tomorrow'," he said.
Neighbour Catherine Delgado, 43, who saw the incident, said about "five or six" cars surrounded one vehicle, blocking off the street in the process.
She said police shouted at the driver to get out of the vehicle, but when she looked he was refusing.