London terror suspects face second day of questioning
Four men are being questioned for a second day by officers investigating what Scotland Yard says is an "Islamist-related" plot.
The men, all aged 20 or 21, were arrested in raids throughout the day on Tuesday which saw one of them Tasered.
The BBC understands one of them is medical student Tarik Hassane, from west London.
Whitehall officials have said the police operation "may have foiled the early stages" of a planned UK attack.
Under terror legislation the men could in theory be held without charge and questioned for up to 14 days.
They were detained under the Terrorism Act on suspicion of being involved in the commission, preparation or instigation of terrorism.
Met Commissioner Sir Bernard Hogan-Howe told BBC London radio it was a "quite serious case".
"It is one of a series of arrests that we have had over the last few weeks which, taken together, for me confirm that the drumbeat around terrorism has changed," he said.
"It's a more intense drumbeat - we are having to be more interventionist and a lot of it is linked back to Syria and Iraq.
"These are arrests that in some way or other have got that sort of link."
Security authorities believe more than 500 British nationals have travelled to Iraq and Syria to fight on behalf of Islamic State and other militant groups.
By Danny Shaw, BBC home affairs correspondent
Finding out what lies behind anti-terrorism investigations is always a challenge, with knowledge shared by only a tight circle of security officials and detectives.
Inevitably, speculation fills the vacuum. The latest arrests in London appear to be related to concerns that a plot was being hatched to target the UK.
What exactly was being planned, if anything, is likely to become clearer after police have questioned the suspects and examined material seized from their homes: computers and phones could be crucial.
Was there a lot of loose talk? Or detailed planning? The Metropolitan Police Commissioner said that at present his officers have to be more "interventionist". That suggests that rather than letting surveillance or a covert operation run, in the hope of discovering concrete evidence, police are stepping in earlier to disrupt activity - just in case.
A number of homes and vehicles were searched by specialist officers in west and central London on Tuesday as part of the investigation.
Mr Hassane, thought to be the 21-year-old who was Tasered during the joint operation by MI5 and the Met, was not injured and did not require medical treatment, police said.
BBC home affairs correspondent Danny Shaw said the dawn raid on Mr Hassane's flat in the Ladbroke Grove area of west London was dramatic and involved a number of firearms officers.
Scotland Yard would not comment on where the four men were now being held.
Officials said one of them had spent time in Syria.
"I don't want to worry the people of London," Sir Bernard added. "We are here to keep them safe and we believe we are on top of things.
"But there are some concerns about the people we are having to arrest."