British authorities said on Tuesday they may have foiled an Islamic State-inspired plot to carry out attacks on the streets of London as police arrested four men in a string of raids across the capital.
The suspects, all aged 20 and 21, were being held for questioning on Wednesday under the Terrorism Act, which allows for detention without charge for up to 14 days.
One of them had recently returned from Syria and was reported to have links with IS, a dynamic which is increasingly generating British alarm over the potential for blowback from the Middle East conflict.
Police revealed few details of the allegations against the men, but media reports suggested they had been under surveillance by police and domestic intelligence agency MI5 for some time.
One line of inquiry is that the men were planning a beheading and other random killings in public places, The Telegraph reported.
The Metropolitan police commissioner, Sir Bernard Hogan-Howe, told the BBC that 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.
"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."
Islamic State fighters in Syria have already beheaded two British and two American hostages, releasing gruesome execution videos in which each killing is followed by the appearance of another captive and the threat that they will be next.
Hundreds of Britons are believed to have traveled out to join IS in Syria and Iraq and security services are concerned that some of those might return to the UK with the intention of carrying out attacks, perhaps emulating the unsophisticated but high-profile killings which have become Islamic State's modus operandi.
In August an IS spokesman, Abu Mohammed al-Adnani, called on the group's supporters to kill Westerners on the streets in retaliation for their governments' involvement in airstrikes, suggesting the possibility of knife attacks — reminiscent of the public murder of British soldier Lee Rigby which horrified Londoners last year.
The government has already confiscated 23 passports of individuals thought to be trying to travel out to join IS, and is planning toughened up measures that could see suspected returning jihadists jailed or even barred from entry to Britain. It has also announced proposals to restrict the movements and speech of extremist preachers.
"You have declared your allegiance — you are an enemy of the UK and you should expect to be treated as such," Prime Minister David Cameron warned last week.
Critics have raised concerns that British involvement in airstrikes is raising the danger to IS hostages, and that the clampdown on extremism will fail to tackle the root causes and only generate more support for Islamist groups.
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