London (CNN) -- 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.