A woman who was killed in front of horrified onlookers was in "exactly the wrong place at the wrong time" when she was crushed by falling masonry, an eyewitness said.

The victim, believed to be in her 20s, was pronounced dead at the scene after being struck by building debris, believed to be a concrete window frame, in Hanover Square in central London.

A man who was delivering electrical cables to the area made the 999 call and said the woman was "flattened" and that he thought he saw her "draw her last breath".

The man, who did not want to be named, said: "I heard this incredible loud bang. The guy next to me said: 'What the hell is that?', pointing. I turned round and the frame was on the ground and I was hoping it was a coat underneath, but then I saw an arm."

He said it took five of his colleagues and around five more workers from the area to lift the concrete frame off the woman.

"I realised it was a person underneath so I phoned 999. The operator asked me to go closer to the victim so I got off the back of my wagon and went over. There was a woman who said she was a doctor who wanted to perform CPR and there was somebody giving her mouth-to-mouth. Her eyes were open and I saw her hand move and then I think I saw her last breath and then the light went out, she'd gone."

The man said ambulance crews arrived around a minute or two later.

An air ambulance, two cycle responders, two ambulance crews and members of the hazardous area response team were sent and extensive efforts were made to resuscitate the woman, a London Ambulance Service spokeswoman said.

The delivery worker who made the 999 call went on: "I knew what the result would be because even if she had survived - how do you recover from that?" Visibly distressed, he added: "You'd have more chance of winning the lottery than that happening to you. She was in exactly the wrong place at the wrong time."

A Health and Safety Executive (HSE) spokeswoman said they were aware of the incident and were "making initial inquiries".