Update: A man has been arrested. Click here for more.
Abby Baafi, who came face to face with the 'bomber' in Tottenham Court Road has said police have told her they fear he could "blow the building up."
The 27-year-old office worker spoke to The Huffington Post UK, saying she was in a cafe with police officers. She said there were no longer any hostages and her colleagues were safe, although police have yet to confirm this.
However shaken Abby said the man had gas cannisters and chemicals, adding: "The police are worried if he ignites them he'll blow the building up."
Earlier the head of training and operations at Advantage, a company which offers HGV courses, said the man had targeted her offices and was currently holding four men hostage. She told The Huffington Post UK: "He just turned up, strapped up in gasoline, gasoline cylinders. Basically he threatened to blow up the offices. Says he doesn't care about his life, doesn't care about anything."
"He was specifically looking for me but I said 'My name's not Abby' and he let me go."
A Met police spokesperson said: "We are in attendance in Tottenham Court Road... There is a 300m cordon in place. The negotiator is on the scene. It's unclear at this stage if there are any other people in the building. There's nobody that's reported any injuries at this time but the incident is ongoing."
27/04/2012 17:45 BST
Police cordons have been lifted around the offices next to the incident
Huff Post UK staff have been allowed back in, things seem to be winding down. We'll bring you more details on this blog as an when we have them.
27/04/2012 17:44 BST
In a statement this afternoon, Commander Mak Chishty said: "We were called at midday to reports of a man causing a disturbance on Tottenham Court Road. He was located on the 5th floor of the building at No. 179.
"Items were thrown from the window onto the street below.
"We were concerned that he may have explosive materials or flammable liquids with him. Our immediate concern was for the safety of the people inside the building and for the general public in the vicinity.
"Specialist Metropolitan Police negotiators were called to the scene, with colleagues from London Ambulance Service, London Fire Brigade, and specialist firearms officers.
"During the course of the incident a number of people who were inside the building left peacefully.
"After contact with our specialist police negotiators the man came out of the building, and was arrested at 15.00hrs. He is now in police custody.
"We are currently searching the premises to ensure that they are safe, and will begin a process of returning the area to normality as soon as possible.
"We would like to thank the local community, the business community and yourselves [the news media] for their co-operation this afternoon."
27/04/2012 15:14 BST
Police said: "We have made an arrest. A man has been arrested and a search of the buildings is underway. The cordon around the area will be removed in due course. There are no indications of hostages involved."
27/04/2012 15:10 BST
@ metpoliceuk :
We have arrested a man at Tottenham Court Road. A search of the building is underway. Further details to follow #TCR
27/04/2012 15:10 BST
Sky reporting a man has been arrested
27/04/2012 15:03 BST
Rajesh Kalia, of Goodge Law solicitors, who works two floors below where the incident was unfolding in Shropshire House, relived the drama.
The lawyer said: "We were in the office and suddenly we were told by the police that there was some problem in the building and asked to evacuate immediately.
"The police were very clear about the instructions. There was no panic or anything but they were very firm and got everybody out of the building very, very quickly.
"I overheard someone say that there was a man in there with some wires coming out of his jacket.
"Now, I don't know how much of that is true but that's the impression they gave us.
"They mentioned something about a flame-thrower, the jacket and wires coming out of it, and that's about it.
"We're two floors below where this was happening.
"We heard some shouting in the stairwell and then were asked to leave by the police so I think the police got there pretty quick."
27/04/2012 14:54 BST
27/04/2012 14:47 BST
It's still unclear if hostages are inside the offices - Sky News have more info
@ skymarkwhite :
Eyewitness tells Sky News 3 people led from stand-off building #tcr claiming there was still a person inside with man at centre of siege
27/04/2012 14:41 BST
Paul Vale, our deputy news editor
"We've been pushed all the way back to Euston road. There are maybe 50 people here by the police cordon. Earlier there were just 100s of people floating around in front of the cordon. They've shut all the underground stations near here."