The fire department said early today that the explosion was caused by a bomb containing 200lb of C-4 plastic explosives.
A group calling itself the Serbian Liberation Front also claimed responsibility for the blast. New Yorkers have hitherto escaped the scourge of international terrorism.
The explosion occurred at midday and by nightfall hundreds were still trapped inside the building on the southern tip of Manhattan.
'We thought it was a bolt of lightning,' said Deborah Sale, a member of the governor of New York's staff, who was on the 57th floor. 'There was a big thunder clap and then the computers surged and the lights flickered out.' Others in the buildings said they thought there had been an earthquake.
A British merchant banker, Peter Stanhope, was trapped on the 85th floor. Speaking by telephone more than two hours after the explosion, he said he and another 100 people on his floor still did not know how they would get out. People headed for the emergency exits but found the lighting had failed, leaving them in total darkness with smoke billowing up from two electrical fires below.
Meanwhile, firefighters smashed windows as high as 40 floors up, trying to ventilate the tower. 'There were aftershocks, and deep rumblings beneath us,' said Scott Frances, a survivor whose face was covered with soot. 'We thought a major earthquake was under way.'
Police and local television stations were deluged with phone calls from people asking how to escape. Several made grim jokes about the 1970s film about a skyscraper fire, Towering Inferno. In the end there was an orderly evacuation down the stairs.
The explosion caused a roof to collapse in the railway station under the buildings, which are the second tallest in the world after the Sears Tower in Chicago.
Police cars and ambulances raced through the streets, bringing confusion to Manhattan as helicopters hovered over the twin towers. The chaos worsened when a second bomb threat forced police to evacuate the Empire State building in midtown Manhattan.
New York's airports were then placed on security alert.
As many as 100,000 people work in the World Trade Centre. A group of visiting schoolchildren were trapped on the 107th floor. A pregnant woman was rescued from the roof by helicopter, but further evacuation was frustrated by low cloud covering the top of the buildings, which rise a third of a mile high above the streets.
There were no immediate estimates of the damage, but the building is expected to be closed while structural damage is assessed.
The blast is believed to have taken place about 200 feet underground, beneath the Vista Hotel, which sits on the plaza beside the two towers. The blast ripped a hole 180ft by 12ft in a wall above the underground station.
More than 200 firemen put out the blaze caused by the explosion and then helped people through the acrid smoke that filled the lower floors.
In Washington, White House officials confirmed that three Secret Service agents had been injured in the blast, and said that a fleet of official cars, including the limousine used by the President when he is in New York, had been damaged.
The fire caused limited damage to some over-the-counter market computers at downtown brokerages and led to the early closing of two commodity exchanges at the World Trade Centre complex, the New York Mercantile Exchange and the Commodity Exchange.
(Photograph omitted)Reuse content