The helicopter clipped a crane attached to one of the tallest residential towers being built in London and then spun out of control, slamming into Wandsworth Road and setting cars and neighboring buildings on fire, according to spokesmen for police and fire brigades. Parts of the crane rained onto the road, with some pieces hitting the back of cars whose drivers ran for cover.
The helicopter crashed just south of the River Thames near the Underground and mainline train station at Vauxhall, and near the British spy agency MI6.
The helicopter's pilot was killed, and an additional person was killed by the crash, police said. Firefighters rescued one person from a burning car. One person remains in critical condition, five were hospitalized and four people were released after emergency officials treated them at the scene, according to police. Officials have ruled out terrorism.
"I thought it was a bomb," eyewitness Andrew Beadle told the BBC. "I literally looked up as I heard this massive noise and saw the helicopter blades go down."
"There was a massive explosion," another eyewitness told the BBC in the minutes after the crash. "There's at least three cars are on fire. ... It's madness, absolute madness. ... There was chaos.''
Video filmed by eyewitnesses shows burning wreckage on the street and a line of flaming fuel. One person yelled to another to get out of his burning car. Black smoke fills the area, a transport hub that is one of the busiest in the city.
The helicopter was on a scheduled flight from Surrey and had been diverted to nearby Battersea heliport, according a Metropolitan Police official, and was flying through low clouds and fog. A warning, known as a Notice to Airmen, advised pilots to be on the lookout for a "high rise jib crane" with a height of 770 feet in central London attached to a 50-story residential building, the St. George Wharf Tower.
"I was 100 percent sure it was a terrorist attack," Allen Crosbie, who works nearby, told The Associated Press. "There was debris everywhere, a ton of black smoke. Parts of the crane, parts of the helicopter. I heard bang, bang, bang -- I presume it was the helicopter hitting the crane and then the ground. People were just panicking. Everyone thought it was a terrorist attack."
"There was a huge array of debris plummeting to the ground," a woman who identified herself as Angela told the BBC. "There were fires breaking out. Plumes of gray smoke getting bigger and bigger."
Emergency services arrived on the scene within five minutes, eyewitnesses said. Pieces of the crane and helicopters fell on neighboring rooftops, causing multiple fires that nearly 60 firefighters put out. At least one car was incinerated by debris.
The helicopter was an Augusta 109, an eight-seater that aviation experts say is capable of flying in poor weather conditions. Police quickly set up a makeshift trauma ward in a Starbucks not far from the crash scene.
"The top of the crane was actually obscured by fog so I didn't see the impact," eyewitness Michael Gavin told the BBC.
"But I heard a bang and saw the body of the helicopter falling to the ground along with pieces of the crane and then a large plume of smoke afterwards."