STORY HIGHLIGHTS
London (CNN) -- Four men were arrested Tuesday in London on suspicion of planning an act of terrorism, one that the city's police commissioner described as "quite a serious case."
Armed police officers assisted in the arrests of the men -- two of whom are 20 years old, while the others are 21 -- according to London's Metropolitan Police. It is rare for police in Britain to execute warrants accompanied by armed officers, unless the threat is deemed serious.
A UK security source said Tuesday that authorities may have foiled a terror plot aimed at Britain in its early stages. Islamic terrorism was "the clear reason" for the plot, according to the source.
Scotland Yard has announced that the four are being held on suspicion related to the "commission, preparation or instigation of act of terrorism." Authorities have not detailed exactly where or when such terrorist attacks might have occurred or what or who they might have involved.
The arrests come nearly two weeks after police arrested at least 10 other men in the United Kingdom on suspicion of terror offenses, including supporting a banned organization and encouraging terrorism.
Metropolitan Police Commissioner Bernard Hogan-Howe, in an interview with BBC local radio, said a lot of recent arrests are "linked back to Syria and Iraq."
That is where the Islamist extremist group ISIS, which calls itself the Islamic State, has waged a bloody and largely successful campaign to take over vast swaths of land -- while terrorizing and, in many cases, killing those civilians who don't subscribe to its extreme brand of Islam in the process.
Two British aid workers are believed to have been beheaded by ISIS, in addition to a pair of Americans. Britain is among the nations that have gone after the Islamist extremist group militarily in Iraq, while the United States and some of its allies are targeting the group inside Syria.
"These are arrests that, some way or other, have that link," Hogan-Howe said of the Syrian and Iraq tie.
10arrests in late September
As of Tuesday night, authorities hadn't released the names of the four men arrested. Nor is it known what their future will hold.
"They have all been taken to police stations in central London and remain in custody," Metropolitan Police said.
Even after the arrests, officers searched other homes and vehicles in west and central London as part of the investigation, authorities said.
Police didn't say whether Tuesday's moves were connected to the September 25 and September 26 arrests of 10 other men. Those men, ages 22 to 51, were detained in London and elsewhere in the country on suspicion of being members of a banned organization, supporting a banned organization and encouraging terrorism.
An 11th man was arrested September 26 on suspicion of assisting an offender.
Police did not give the identities of those arrested in September nor name the banned organization concerned. But the UK's Press Association news agency cited sources as saying radical British cleric Anjem Choudary was among those arrested.
Choudary, who was a co-founder of the banned UK Islamist group Al Muhajiroun, told CNN in August that the world had been split into two camps.
There's a "camp which believes that sovereignty and supremacy belongs to God. They are the Islamic State, at the head of which is Abu Bakr al-Baghdadi," Choudary said. "In the other camp you have those people who believe sovereignty and supremacy belongs to man. At the head of that camp is Barack Obama."
"I believe this Islamic State will spread, rapidly, and I believe it will be in Europe and even America within decades," Choudary said.
The self-proclaimed Islamic State, commonly referred to as ISIS or ISIL, is a Sunni Muslim extremist group that has captured parts of Syria and Iraq for what it says is its new Islamic caliphate.
The British military recently joined a U.S.-led bombing campaign against ISIS in Iraq. The United States and some Middle Eastern nations also are conducting airstrikes against ISIS in Syria.
UK authorities believe at least 500 British citizens have gone to Iraq and Syria, many of them to fight with ISIS and other Islamist groups -- and that most will try to return, bringing their extremist views with them.
The UK Home Office raised its terror threat level in August from "substantial" to "severe." Days later, the government announced new measures to combat the threat from Islamist extremism, including the banning of Britons' coming home once they have joined with jihadists abroad.
CNN's Victoria Eastwood reported from London and Jason Hanna wrote from Atlanta. CNN's Greg Botelho, Laura Smith-Spark, Max Foster, Andrew Carey and Paul Cruickshank contributed to this report from London.