"It was instantly clear from the amount of debris in the air that it was either a terrible accident or a bomb. Seconds later there was another explosion a few hundred yards away."

He said it took a while for what had happened to register. But the horror soon became clear.

Jupp, who works for a Boston-based company that makes consumer products in China, said marshals and runners struggled with barriers to let police and fire vehicles through.

"There were several scenes of officials desperately trying to pull away safety barriers so that the emergency services could get to the injured people," he said.

"This was all taking place while there was still a great threat of even more bombs going off."

The explosions occurred about four hours and nine minutes after the event started.

Jupp, who regularly takes part in marathons, said: "Runners came through the ranks crying and tensions started to rise.

"The area near the finishing line was packed with families cheering on their loved ones. It's despicable what happened."

Three people were killed in the attacks, including an eight-year-old boy.

Among the 170 wounded in Monday's attack was a student from China.

Zhou Danling, who is studying actuarial science at Boston University after graduating from Wuhan University, was in stable condition last night after undergoing surgery, state television reported.

Foreign Ministry spokesman Hua Chunying said officials at the Chinese consulate in New York visited Zhou in hospital. State television also reported last night that another mainland student was missing.

Others among the 50 Hong Kong residents registered for the run, none of whom was injured, told of the confusion and fear.

Viola Lo had just crossed the finishing line and was a few hundred metres away from where the bombs went off.

Despite the chaos, Lo said she did not realise how serious the incident was until she got back to her hotel. She said: "People were scared. There were many police cars and ambulances."

Mobile phone services started to fail, leaving many runners unable to contact loved ones.

Jacqueline Walsh, a managing director with accountancy firm Borrelli Walsh who lives in Clear Water Bay, was worried about her parents, who were waiting at the finishing line. She had got to within 800 metres of the finish only to find the route blocked off following the explosions.

"When I heard what had happened I desperately tried to phone my parents, but couldn't get through," Walsh, 44, said.

"That was the worst time for me - not knowing if they were both OK."

Walsh later found out that, although in a state of shock, her parents were safe and well.