Christopher Jen was at Vauxhall Tube station on his way to his marketing job when he spotted smoke rising nearby.
"We could hear all the sirens and you could smell the smoke as the train went past the site," he said in a submission to CNN's iReport. "The station announcers did tell us that there was a helicopter crash at Vauxhall and the engineers had to go check the tracks before we could leave."
Reporters allowed through the cordons to the deadly crash scene say little can be seen other than a dark spot where the helicopter hit the ground.
A Berkeley Group spokesman said the firm was giving its full support to the emergency services after its crane was hit.
"Our thoughts at this time are with the friends and families of those killed in this tragic incident," he said.
Firefighters are now working with contractors to make the mangled crane safe.
Prime Minister David Cameron thanked the emergency services for their "rapid and professional response."
Many helicopters follow the Thames as a flight route through the city. This helicopter appeared to be flying farther south than usual, Whipps said.
No cause has yet been given for the crash, but visibility was poor in London on Wednesday morning, with fog and a very low cloud ceiling.
Chris Yates, an aviation security expert, told CNN it was likely there would be questions about whether the helicopter should have been flying in these conditions.
Under safety regulations, tall buildings must have navigation lights on top to make them visible to low-flying aircraft, he said.
Helicopters are not supposed to fly within 500 feet of tall structures in central London, he said.
The London Heliport is near the crash site, on the banks of the Thames, at Battersea.
The Air Accidents Investigation Bureau, part of the UK Department for Transport, has sent a team to investigate the crash.
The Civil Aviation Authority says helicopters in London have only limited radar help and fly mostly using visual reference points on the ground.
Certain designated routes for helicopters are in place to "provide maximum safety by avoiding flying over built up areas as much as possible," it says.
Nearly 170 flights followed the central London route along the Thames in December, its figures show. About 250 flights arrived and departed from the heliport at Battersea in the same month.
Wednesday's crash caused major disruptions 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.
The Nine Elms area between Vauxhall and Battersea has been earmarked as a major regeneration site, with plans to create 16,000 new homes and 25,000 new jobs.