The Guardian obtained this footage of Ian Tomlinson at a G20 protest in London, shortly before he died. It shows Tomlinson, who was not part of the demonstration, being assaulted from behind and pushed to the ground by baton-wielding police guardian.co.uk

Dramatic footage obtained by the Guardian shows that the man who died at last week's G20 protests in London was attacked from behind and thrown to the ground by a baton–wielding police officer in riot gear.

Moments after the assault on Ian Tomlinson was captured on video, he suffered a heart attack and died.

The Guardian has handed a dossier of evidence to the police complaints watchdog.

It sheds new light on the events surrounding the death of the 47-year-old newspaper seller, who had been on his way home from work when he was confronted by lines of riot police near the Bank of England.

The submission to the Independent Police Complaints Commission (IPCC) includes a collection of testimonies from witnesses, along with the video footage, shot at around 7.20pm, which shows Tomlinson at Royal Exchange Passage.

The film reveals that as he walks, with his hands in his pockets, he does not speak to the police or offer any resistance.

A phalanx of officers, some with dogs and some in riot gear, are close behind him and try to urge him forward.

A Metropolitan police officer appears to strike him with a baton, hitting him from behind on his upper thigh.

Moments later, the same policeman rushes forward and, using both hands, pushes Tomlinson in the back and sends him flying to the ground, where he remonstrates with police who stand back, leaving bystanders to help him to his feet.

The man who shot the footage, a fund manager from New York who was in London on business, said: "The primary reason for me coming forward is that it was clear the family were not getting any answers."

The Guardian's dossier also includes a sequence of photographs, taken by three different people, showing the aftermath of the attack, as well as witness statements from people in the area at the time.

A number of witnesses provided time and date-stamped photographs that substantiate their accounts.

Some said they saw police officers attack Tomlinson.

Witnesses said that, prior to the moment captured on video, he had already been hit with batons and thrown to the floor by police who blocked his route home.

One witness, Anna Branthwaite, a photographer, described how, in the minutes before the video was shot, she saw Tomlinson walking towards Cornhill Street.

"A riot police officer had already grabbed him and was pushing him," she said.

"It wasn't just pushing him – he'd rushed him. He went to the floor and he did actually roll. That was quite noticeable.

"It was the force of the impact. He bounced on the floor. It was a very forceful knocking down from behind. The officer hit him twice with a baton when he was lying on the floor.

"So it wasn't just that the officer had pushed him – it became an assault.

"And then the officer picked him up from the back, continued to walk or charge with him, and threw him.

"He was running and stumbling. He didn't turn and confront the officer or anything like that."

The witness accounts contradict the official version of events given by police.

In an official statement on the night of Tomlinson's death, the Metropolitan police made no reference to any contact with officers and simply described attempts by police medics and an ambulance crew to save his life after he collapsed – efforts they said were marred by protesters throwing missiles as first aid was administered .

The force said officers had created a cordon around Tomlinson to give him CPR.

"The officers took the decision to move him as during this time a number of missiles - believed to be bottles - were being thrown at them," it said.

Yesterday, the IPCC began managing an investigation by City of London police into the circumstances of Tomlinson's death after the Guardian published photographs of him on the ground and witness statements indicated he had been assaulted by police officers.

The IPCC commissioner for London, Deborah Glass, said: "Initially, we had accounts from independent witnesses who were on Cornhill, who told us that there had been no contact between the police and Mr Tomlinson when he collapsed."

"However, other witnesses who saw him in the Royal Exchange area have since told us that Mr Tomlinson did have contact with police officers.

"This would have been a few minutes before he collapsed. It is important that we are able to establish as far as possible whether that contact had anything to do with his death."

The IPCC added that Tomlinson was captured on CCTV walking onto Royal Exchange Passage.

"This is the aspect of the incident that the IPCC is now investigating," it said.

It was here the video was shot. A post mortem carried out by a Home Office pathologist last Friday revealed Tomlinson died of a heart attack.

Prior to seeing the dossier of evidence, Tomlinson's family said in a statement: "There were so many people around where Ian died, and so many people with cameras, that somebody must have seen what happened in the Royal Exchange passageway.

"We need to know what happened there and whether it had anything to do with Ian's death.

"We know that some people who were at the protest may not feel comfortable talking to the police.

"People are putting pictures on the internet, writing on blogs and talking to journalists. But we really need them to talk to the people who are investigating what happened."

• The Guardian's Ian Tomlinson video is on YouTube if you wish to embed it on your website or blog.