A helicopter crashed into a crane on top of one of Europe's tallest residential blocks in central London on Wednesday, killing two people as it burst into flames and threw plumes of smoke into the foggy air.
Police said there was nothing to indicate that the crash on the south bank of the River Thames was linked to an attack on London, where 52 commuters were killed in rush hour suicide bombings in 2005.
"There was a really loud bang," said Julie Marsden, who works in an office building near the crash site which is close to landmarks such as the headquarters of Britain's MI6 international intelligence agency and the Houses of Parliament.
"We saw the crane fall to the ground and this massive plume of black smoke," Marsden told Reuters.
London's police Commissioner Bernard Hogan-Howe said there were 11 casualties including two dead and one critically injured. The fire service said it had rescued a man from a burning car near Vauxhall train station, a major commuter station on the south side of London, shortly after 0800 GMT.
Hogan-Howe said the helicopter had been diverted to a helipad in Battersea, near the crash site. He declined to give further information until relatives had been informed.
"We're treating it as a major incident," he told the London Assembly.
Rezart Islami, 27, a construction worker from Kosovo who had been on a nearby site, said he saw the helicopter flying fast up the river before it smashed into the crane, span out of control and hit the ground bursting into flames some seconds later.
The crane fell and hit two cars. "I was shocked, it was spinning around and lost control," he told Reuters.
Another witness, Edmir Pishtar, who was in a van outside the building site, said he saw half the crane crash down and cut into two cars on the road. He later spoke to the crane operator who was about to get inside the crane cab.
"He was literally shaking because he was getting ready to climb into the crane and he was late."
The circular block - The Tower, One St George Wharf - is described on its website as the epitome of luxury London living, with 360-degree views across the capital and over the Houses of Parliament.
Builder Brookfield Multiplex said the tower, which is not occupied, is 52 floors or 185 meters (200 yards) high and has 212 luxury apartments. Media reports in recent years have suggested the Penthouse apartments could go for as much as 50 million pounds ($80 million).
A Reuters correspondent at the scene said tangled bits of crane could be seen hanging off the side of the tower, the top of which was still shrouded by low cloud. TV footage showed flaming wreckage strewn across a road.
"Fortunately for us we have done a full headcount and there are no injuries or fatalities among anyone on the site," said Tony Pidgley, chairman of the tower's developer Berkeley Group.
"The crane driver normally starts at eight o'clock but unusually, today of all days, was late."
He said it was too early to speculate on the cause of the crash but helicopters should normally fly 500 feetabove tall structures.
Police were questioning witnesses in the area and dozens of emergency vehicles were in attendance, Reuters reporters said. London train and underground train services were working as usual.
"There's nothing to suggest any terrorism link," a spokesman for London's Counter Terrorism Command said. The fire service said eight fire engines and 60 firefighters were on the scene.
Helicopters in London are generally supposed to fly along the River Thames but London City Airport said its flights had been disrupted due to low visibility.
The Department of Transport's crash investigation unit said it was preparing the launch an inquiry into the incident.
($1 = 0.6215 British pounds)