A trainee doctor dubbed 'The Surgeon' tweeted: 'Oi lads... I smell war' shortly before he was arrested on suspicion of planning an Islamic State-linked plot to bring murder to the streets of Britain.
Tarik Hassane, 21, was Tasered as MI5 and Scotland Yard swooped on his flat on the Princess Alice Estate in Ladbrooke Grove, West London, yesterday morning.
The medical student is believed to be among four men in their early 20s from London held by Scotland Yard and the security services.
Officials swooped amid heightened fears that fanatics who have returned from Syria are planning a high-profile gun attack or a beheading.
It is understood to be the first plot in Britain that is allegedly directly linked to the Islamic State terrorists responsible for the murders of British hostages Alan Henning and David Haines.
Hassane is nicknamed 'The Surgeon' because he is training to become a doctor and was arrested by armed police who rushed his family home on an estate in North Kensington, where he lives with his mother and sister.
Witnesses said a team of armed officers dressed in all black stormed the house at 4.30am yesterday after throwing in stun grenades.
Father-of-three Michael Mwangaa, 32, described seeing a heavy police presence around Princess Alice House on Tuesday morning in the hours after MI5 agents swooped on the council estate.
He said: 'I heard two or three loud bangs morning at around 5am, but I thought it could have been a gun - I didn't want to go out and have a look.
'Then when I was taking my two eldest kids to school we came out and saw all of the police. There was a van full of officers in uniforms, and a lot of undercover cops.
'They were coming in and out of the estate. A few had bags with them, but they were keeping a low profile.
'When I found out it was Tarik I was shocked. I couldn't believe he would want to do something like this'.
Hassane had hoped to study at Kings College hospital in London but failed to get the grades, he admitted online.
Rather than accepting an offer to do biomedicine he decided to become a medical student in Khartoum Sudan checking with the General Medical Council that the qualifications would be accepted in the UK.
On Twitter he posted pictures of his medical text books and said: 'This makes A Levels look like a joke.
In one post he denied rejecting Kings because it was kufr, a slur meaning non-Islamic.
He says: 'Got an offer from Kings to do medicine, missed out by a couple grades.
'Had the choice of doing biomed but that don't fit in my future plans. Very close friend of mine already completed his first year in Sudan. Checked the uni with the 'General Medical Council' and they said a degree from their (sic) is recognised in the uk. So still get to study medicine + it's recognised world wide + in a Muslim country.'
He also disapproves of women wearing make-up.
In another post he says: 'Wearing make up outside of the home such as foundation/lip stick etc isn't really permissible. And it's better that sisters avoid these things.
'Covering up has been enjoined upon women for a reason (I.e covering their beauty except for their husbands/in front of their mahrams) so wearing make up kind of defeats the purpose of hijab.
There have been repeated warnings from David Cameron and security officials that ISIS (also known as IS or ISIL) has Britain in its sights.
Senior sources said one of the four men was a suspected jihadist, who had recently returned to the UK from Syria.
The Metropolitan Police said the men were arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.
Sources said the planning was at an early stage but public safety had been paramount in the decision to act.
Last month, raids were conducted throughout Australia amid intelligence that IS were planning 'demonstration killings' there, reportedly including a public beheading.
Britain already has terrible experience of such an attack, although the two Islamist maniacs who beheaded Lee Rigby outside Woolwich barracks last year were not linked to Syria.
The raids in Ladbroke Grove come at a time when police activity is at its highest level since the July 7 bombings in 2005.
Officials are particularly worried by the prospect of a so-called 'lone wolf attack', which requires little planning and could be carried out with a knife or firearm.
Police have stepped up armed patrols and searches, while specialist officers have been visiting extremist sympathisers and those who have been to Syria.
An Islamic State video released online last week urged fanatics in the UK to strike.
It showed a British jihadist, believed to be Omar Hussein, a former security guard at a Morrisons supermarket, calling on 'all the brothers in the UK' to rise up.
The militant urged fellow Muslims to travel to Syria and Iraq to join the extremist group in its self-proclaimed caliphate.
For those who cannot make the journey, Hussein said: 'You can cause terror right from within.'
Security services and politicians have long warned of the prospect of an IS plot here.
Earlier this year, David Cameron said: 'The people in that regime, as well as trying to take territory, are also planning to attack us here at home.'
In August, the threat was raised from substantial to severe – meaning that an attack is highly likely.
One case, which is due to be heard this month and is subject to reporting restrictions, involves two men who have been charged with planning terrorist offences in this country.
Other alleged plots have been uncovered in France, Italy and the Netherlands.
In May, Mehdi Nemmouche, 29, who had fought with IS in Syria, was arrested after three people were shot dead in a Jewish museum in Brussels. Police said he had in his possession a Kalashnikov rifle, a handgun and a sheet emblazoned with IS logos.
