Even from his sixth-floor office at Hyflux Innovation Centre in Bendemeer Road, Mr Foo Suan Wang could hear the "very loud, terrible scream" from the streets below.
He rushed to the nearest window, where he filmed a remarkable display of public spiritedness yesterday morning as more than 30 passers-by rushed to lift a trailer truck to free a man pinned underneath.
The man, believed to be a 35-year-old South Korean, escaped with a leg fracture and underwent surgery last night at Tan Tock Seng Hospital, where he was taken by the Singapore Civil Defence Force.
Police confirmed that the 25-year-old lorry driver was arrested and investigations are ongoing.
The 1 and a 1/2-minute video taken by Mr Foo, a senior manager at healthcare company Medtronic, has received more than 177,000 views and 4,100 shares on Facebook - attracting numerous comments praising the passers-by who responded.
"We are One Singapore!!!", wrote one user, while another commented, "faith in humanity restored".
The accident occurred around 11.45am at the junction of Bendemeer Road and Boon Keng Road.
It is not clear how the man ended up under the truck, but Mr Samsuddin Musha, who drove past the scene, believes he was hit while crossing the road and was lucky to survive. "The man, who was well-dressed in a blazer, was lying prone, face-down under the truck. There was a pool of blood," said Mr Samsuddin, a driver at nearby Nishimatsu Construction.
A Filipino design engineer, who wanted to be known only as Dennis, said he was on his way to lunch from his office at German company Continental across the road, when he and his colleague heard someone moaning from 200m away.
"I saw the truck driver in hysterics. When I saw two older men calling over people to lift the truck, we rushed over," he said. When the truck had been lifted high enough, he went under it to help pull the injured man out despite the risks involved. "Someone shouted to get the man free, and I did it instinctively. I was afraid those lifting the truck might get tired, so I made sure to keep as low as possible," he said. "An Indian man helped me. The two of us managed to pull him out. It happened very quickly."
He said none of the helpers tried to turn the victim face up, as they were afraid of injuring him.
"You don't see this every day, people from all walks of life coming spontaneously to help," said Mr Foo, 50. "Well done, Singapore."
Additional reporting by Lim Yi Han