According to Abby Baafi, 27, 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.
Armed police and explosives officers have been called to Tottenham Court Road in central London after a man entered a building and, according to one witness, threatened to blow himself up.
The Metropolitan police increased the cordon around the area from 100 to 300 metres two hours after the incident began around midday. They were called to the offices of Advantage HGV after receiving reports that a man had entered the building and was throwing computers and other equipment from the window.
A police source said the man was "causing concern" but there was no confirmation that he was holding hostages inside the building. A police negotiator was called to the scene "as a precaution".
Rumours that the man had canisters strapped to his body were not confirmed by the police.
Scotland Yard said the man had been moving about inside the building but was now on the fifth floor. A spokesman said the police had received no reports of gunshots being fired in the area.
In a video posted on YouTube by Stephen Hull, an editor with the Huffington Post, a witness who works at the transport and logistics firm said the man walked into the office with gas canisters strapped to his body.
Abby Baafi, the head of training and operations at Advantage HGV, said she recognised him as a customer of the company, which provides HGV (heavy goods vehicle) training courses.
"He turned up, strapped up with gasoline cylinders, and threatened to blow up the office," Baafi said at the scene. "He said he doesn't care about his life. He doesn't care about anything, he is going to blow up everybody."
She said the man had failed an HGV training course and wanted his money back.
Scotland Yard said in a statement: "Officers are in attendance at an incident in Tottenham Court Road, where a 50-year-old man is causing a disturbance.
"Police were called at 11.59am on Friday 27 April to an office building on Tottenham Court Road. Items, including electrical equipment, have been thrown out of a fifth-floor window.
"A 300-metre cordon has been put in place and a negotiator is on scene. We would advise people to keep clear of the area due to congestion."
The statement added that no injuries had been reported and it was unclear if there were other people in the building.
Sarah O'Meara, who works for the Huffington Post, said the media company had evacuated its offices in nearby Capper Street after being alerted by a woman who ran into the building.
"A woman ran in off the street saying: 'There is a guy with a bomb and he is threatening to blow himself up', and that we needed to evacuate," she said.
"Everyone got out. The police have been moving people back street by street."
Images on Twitter showed various items being thrown from the building, including computer monitors and piles of paper.
O'Meara said the atmosphere had been "tragi-comic" until the police arrived and it turned serious.
"He was throwing stuff out of the windows -- it looked like someone with a grievance," she said. "But then the police arrived and started telling everyone: 'This is serious, this is for your own safety. He has got gas.'"