LONDON (AP) _ At least one explosion went off at the headquarters of Britain's MI6 intelligence agency headquarters in central London late Wednesday, causing some damage but no casualties, police said.

The blast brought firefighters, police and ambulances to the site on the south bank of the River Thames, and police closed the area around the headquarters. Witnesses said they heard two explosions.

A police constable at the scene who refused to give his name said an explosion had occurred and that the cause was unknown.

A Scotland Yard spokesman, referring to the incident only as ``a possible explosion,'' said there was ``minor damage to the building, believed to be on the eighth floor.'' He said explosives officers were at the scene and that he had no information on the likely cause of the incident.

Foreign Secretary Robin Cook, whose department is responsible for MI6, said there were no casualties and expressed relief.

The building looked intact from a distance of several hundred yards, but witness James Trott, 42, said there was shattered glass on the river side of the building.

The big modern structure near the Vauxhall Bridge across the Thames was featured in the James Bond movie, ``The World is not Enough'' and in one scene was shown being blown up by a terrorist bomb.

``We just heard two God-almighty bangs, a loud one first, and then another one,'' said Theresa Harding, 68, who lives nearby. ``We honestly thought it was fireworks.''

One witness, Andrew Preece, told BBC News 24 he was driving home when he saw a flash of light coming from the top of the MI6 building.

``It looked as of it was internal from the center of the building but the flash of light was from the top of the building,'' he said. ``It was followed by a large bang and the ground seemed to shake and my car was shaking.''

Witness Alex Frank said he was in his his bedroom across a railway from the building when he heard two explosions and saw ``massive white clouds.'' He said the blasts ``were loud enough to shake the building I'm living in.''

A London Fire Brigade spokesman said crews were called to the area at 9:25 p.m. Train service was halted from nearby Waterloo station.

Stephen Dorrill, an expert on MI6, told BBC News 24 he would be ``extremely surprised'' if anyone had managed to get inside the building and plant explosives. If that had happened, it would be ``an absolutely major security breach,'' he said.

Earlier this year, the Foreign Office said it was tightening security procedures after reported lapses.

In March, a laptop computer that contained missing training information for one of Britain's spy agencies was mislaid and recovered by police two weeks later. News reports suggested that an M16 agent left the laptop in a taxi after spending a night drinking at a bar near the agency's headquarters.

MI6, formed in 1921, is responsible for foreign intelligence. It is thought to cooperate closely with the domestic security service MI5 in keeping track of Irish terror groups.

The high-tech spy headquarters bristles with security features. Much of the complex is below street level to protect the most sensitive areas from terrorist attacks.