A medical student is among four men being questioned by counter-terror police on suspicion of being involved in a potentially "significant" Islamist terrorism plot.

One of the suspected extremists is 21-year-old Tarik Hassane, who lives near Ladbroke Grove in west London, the Press Association understands.

At least one of the suspects is believed to have travelled to Syria and one line of inquiry is to establish any possible links with Islamic State (IS), the extremist group behind the beheading of British aid worker Alan Henning.

Hassane and three others, aged 20 to 21, were arrested yesterday in a series of raids at addresses across London, during which one suspect was Tasered by police.

Counter-terror officers believe the raids were an "early disruption" of what could have turned into a "significant plot".

In a conversation on social networking site Ask.fm, "Tarik Hassane from Ladbroke" claims to be studying medicine in Sudan after failing to meet the grades for an offer to study at King's College London.

It is understood that Hassane's mother is Moroccan and works in a school.

The front door of Hassane's property showed signs of forced entry and has been boarded up.

One neighbour, who said she did not want to give her name for fear of reprisals, said she had heard that "police took the floor up" in the property.

"He was a really nice guy who loved his football," she said. "He used to be a bit of a Jack the lad but then he kept going to Morocco and around two years ago he started covering his tracksuit with a white dress and then the Jesus creepers came in and he grew a beard. He started behaving shy with people he knew all his life. Everybody noticed it."

Neighbours heard bangs at around 5.30am and police reassured those nearby not to panic and return to their flats.

One neighbour, who gave his name only as Mohammed, said at first he thought there had been a shooting.

"If it's the guy I think it is, he's a good boy, good-looking and well-educated," he said.

"The problem around here with young people is drugs - not extremism - and he did not seem to be involved with the drugs. I haven't seen him around for a while but he seemed like a normal kid."

The arrests come less than five weeks after the terror threat level in the UK was raised from substantial to severe, meaning a terrorist attack is ''highly likely''.

The Joint Terrorism Analysis Centre (JTAC) raised the level against a backdrop of increasing concerns over hundreds of aspiring British jihadis travelling to Iraq and Syria to learn terrorist ''tradecraft''.

Fears heightened after IS posted a series of videos online showing the separate murders of US journalists James Foley and Steven Sotloff and two British aid workers, David Haines and most recently Mr Henning.

Footage claiming to show Mr Henning's murder appeared on the internet last Friday, just days after the UK joined US-led air strikes against the terrorists in Iraq.

In addition, an apparent IS fighter with a British accent appeared unmasked in another film encouraging "brothers" in western countries to "rise up" and commit acts of terror in their home countries.

IS, led by fanatic Abu Bakr al-Baghdadi, has taken over large swathes of Iraq and Syria and attracted thousands of foreign jihadists to its cause, including around more than 500 Britons.

America's Federal Bureau of Investigations (FBI) has launched an appeal for the public's help in identifying individuals who have travelled - or are planning to travel - overseas to engage in combat alongside terrorist organisations.

The law enforcement agency said it was seeking information about the identity of an English-speaking man, with a North American accent, seen in an IS propaganda video released last month.

The FBI also repeated its claim that authorities "now know" the identity of the executioner claiming to have killed the four western hostages, including Mr Henning.

Yesterday, the Prime Minister demanded that former Guantanamo Bay detainee Moazzam Begg reveal what he knows of the terror group.

The appeal to Mr Begg, who had terror charges against him dropped last week, came after he said he believed he knew the identity of senior members of IS who held Mr Henning and others.

But he said he did not know the identity of the suspected murderer - who has been nicknamed "Jihadi John".

A statement from Scotland Yard yesterday said all four men were arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism under the Terrorism Act 2000.

A number of residential addresses and vehicles were searched by specialist officers in west and central London as part of the investigation.

The statement added: "These arrests and searches are part of an ongoing investigation into Islamist-related terrorism."

Under terrorism laws, police officers can hold the men for questioning for 48 hours before they have to apply to a magistrate to detain them for longer - but they could be held without charge for up to 14 days.

Commenting on the arrests, Metropolitan Police commissioner Sir Bernard Hogan-Howe said: "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."