London: Scotland Yard officers arrested four men here today on suspicion of terror offences as part of an investigation into an alleged Islamist terrorist plot.
Officials told the BBC the arrests "may have foiled the early stages" of planning an attack in the UK.
The men - two aged 21 and two aged 20 - were held by counter-terrorism police and a Taser stun gun was also used as part of the operations.
They were arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism, Scotland Yard said.
Officials said one individual had spent time in Syria.
There was no word on alleged targets or how far advanced the alleged plot was.
Those arrested were taken to police stations in central London and remain in custody.
A number of residential addresses and vehicles are being searched by specialist officers in west and central London as part of the investigation.
Firearms officers assisted in one of the arrests but no shots were fired.
A 21-year-old who was tasered by officers was not injured and did not require medical treatment, police said.
Prime Minister David Cameron last month revealed tougher measures he wants to impose on Britons planning to travel and fight with Islamic State (IS) militants in Syria and Iraq, plus those who could return to launch attacks on home soil.
Some 500 British jihadists are estimated to be fighting in the two countries.
British police made 69 arrests linked to fighting in Syria in the first half of 2014, a rate five times higher than in 2013.