Two people were killed and two others injured Wednesday when a helicopter crashed into a crane atop a new building in central London, police said.
The chopper plunged to the ground after the impact, sending smoke and flames billowing into the sky at the height of the morning rush hour.
The condition of the two injured people is not yet known, the police said.
A CNN producer at the scene saw an enormous fireball and clouds of black smoke coming from the site of the crash in Vauxhall, south of the River Thames in central London.
Emergency workers rushed to Wandsworth Road, a busy route through Vauxhall.
London's Metropolitan Police said the the alert was raised at about 8 a.m., when many commuters would have been traveling through the area.
Many emergency vehicles were on the scene and roads in the area were cordoned off.
CNN producer James Whipps, who was in an office in the area, heard the sound of the helicopter rotors suddenly cut out.
He did not see the impact but as he looked though a window, he "suddenly saw this enormous fireball, black smoke, shoot up from the side of a building."
He said the helicopter wreckage appeared to have landed on two cars.
The crane on top of the tall building, which is badly bent from the impact and dangling from the top of the building, he said.
London's fire service said firefighters were responding to "reports of a crane in a precarious position" at St. George Wharf.
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The main fire appeared to be swiftly brought under control but smoke was still rising, the CNN producer said.
Many helicopters follow the Thames as a flight route through the city. This helicopter appeared to be flying further south than usual, he said.
Vauxhall, not far from Westminster, is where a new U.S. Embassy is being built.
The area, which is a mix of industrial, business and residential use, is also home to the headquarters of the UK intelligence service, MI6.
No indication of the cause of the crash has yet been given but visibility was poor in London on Wednesday morning.
The incident is likely to cause major disruption for many people on their way to work.
Vauxhall, with a rail station, Underground station and large bus station is an important transit point for those traveling from the south to other parts of London.