Counter-terrorism officials said one of the group may have had access to weapons and this was reflected by the fact armed officers led the raid on his address. During that raid an officer fired an electric current from a Taser intended to subdue a suspect.
The four men, aged 20 to 21, were being questioned at police stations in central London.
The Metropolitan police said officers from its counter-terrorism command had arrested the four “on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism”.
It is not believed that explosives were involved in any plot, although searches were described as at an early stage.
A Whitehall source told the Press Association that officers believed the raids were an early disruption of what could have been a “significant plot”.
It is understood that one of the four has a connection to Isis, the Islamist extremist group that has captured large swaths of territory in Iraq and Syria and has been responsible for the beheading of Western hostages, including two Britons.
Foreign policy and security analysts have been warning that there could be a retaliation in the UK in response to British fighter planes joining the US and some Arab states in bombing raids on Isis in Iraq. An Isis member last week called in a video for Muslims in Britain to rise up to cause terror in the country.
However, the arrests are not connected to the Isis member with a British accent shown in videos of the beheadings. Although the man was masked in the videos, the FBI claims he has been identified.
The four men arrested were all resident in Britain. One of the addresses was in west London and the rest were in central London. Scotland Yard said: “One of the men aged 21 was Tasered during his arrest. He was not injured and did not require medical treatment.
The Met added that officers from the counter-terrorism command – SO15 – “were assisted in the execution of warrants at one of the addresses by officers from the specialist firearms command (SCO19). No shots were fired.
“A number of residential addresses and vehicles are being searched by specialist officers in west and central London as part of the investigation. The searches are ongoing. These arrests and searches are part of an ongoing investigation into Islamist-related terrorism.”
Surveillance led counter-terrorism officers to assess that one of the men may have access to weapons, hence the decision to send armed officers to one of the addresses. It was at that address that the Taser was used.
The raids followed a pre-planned operation, with at least some of the men having been subject to monitoring by counter-terrorism investigators for some time.
Following a joint meeting between SO15 and MI5 at an executive liaison group, the decision was made to disrupt any plot. Counter-terrorism investigators believed they had enough material to stage arrests.
The decision to disrupt a suspected plot is usually taken because it is believed there is too great a chance of an attack being staged or because investigators believe they have enough evidence to test their suspicions through the courts.
However, some past high-profile terror arrests have been based on intelligence that turned out to be inaccurate, and have led to accusations that police and MI5 have ramped up the nature of possible plots.
The fear of a terrorist attack being carried out by people with connections to Syria or Iraq led the Joint Terrorism Analysis Centre (JTAC) to recommend raising the terrorist threat level in August to severe, its second highest level.
Announcing the decision to change the threat level the home secretary, Theresa May, said it was “related to developments in Syria and Iraq, where terrorist groups are planning attacks against the west”.
Counter-terror investigators in both the police and MI5 have been facing an increasing workload as they assess and investigate potential threats from people with connections to several hotspots around the world.
The threat from Isis extremists is the current main source of concern, but there remains anxiety about those with connections to alleged violent extremists in Syria and al-Qaida linked groups there, as well as al-Shabaab in Somalia, and also al-Qaida inspired terrorists with links to Pakistan.
The security services assess how dangerous terror suspects are, but in the two instances in the modern era which saw people murdered – the July 2005 attacks in London and last year’s attack on the soldier Lee Rigby – the violent jihadists responsible had slipped through the net.
After Tuesday’s arrests the terrorist threat level remained unchanged at severe. This means that JTAC, which is housed within MI5, has advised ministers that an attack is highly likely.