The Metropolitan Police faced fresh allegations of brutality last night after it emerged that a man who died at the G20 protests may have been attacked by riot police three times.

Ian Tomlinson was walking home from work when he was hit and pushed over by an officer in an apparently unprovoked attack. Minutes later the 47-year-old lay dying on a London street.

Last night, as it was revealed that the police officer concerned had come forward one week after the events, shocking new claims emerged that the violence meted out to Mr Tomlinson was even more severe than first thought.

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A video had already emerged showing the newspaper seller being shoved forcefully to the ground by police as he walked with his hands in his pockets. But a photographer told The Independent that the incident was merely the third of a series of police attacks on Mr Tomlinson, before he died of a heart attack. Anna Branthwaite said that he had appeared desperate to get away during an earlier "unprovoked" attack in which police lashed at him with a baton. Last night, the Independent Police Complaints Commission (IPCC) announced it was taking over responsibility for a criminal investigation and was still trying to trace other officers involved. Mr Tomlinson's family said they were "shocked and sickened" by the video and would not rest until they had answers.

Ms Branthwaite told this newspaper how she witnessed two initial attacks further up the street as Scotland Yard's riot police swarmed the area.

"The dog handlers [believed to be City of London Police] were just starting to sweep the street and form a police line when Ian Tomlinson arrived. I saw a riot police officer charge him from behind and propel himself forward with his body weight," she said.

"Mr Tomlinson was on the ground and I saw him [the policeman] stand over him with a baton, hitting him twice. He was completely taken by surprise. He didn't know what hit him."

She added that he tried to get to his feet. "His natural instinct was to get up to run. The same riot police officer then grabbed him and ran with him for two strides and threw him forward." She then lost sight of Mr Tomlinson as she was hauled away by police. She was adamant that the filmed attack occurred seconds after the ones she witnessed. "He was not engaging with them [the police]. He was not taunting them. He was not shouting. It was completely unprovoked." Ms Branthwaite has made a statement to the IPCC, which will now pursue the investigation independently.

The shadow Home Secretary Chris Grayling described the revelations as "alarming" and said they left, "big questions to be answered by the police".

The gravity of the situation facing Scotland Yard was reflected in a statement yesterday by its Commissioner, Sir Paul Stephenson: "My thoughts are with Mr Tomlinson's family at this time. The images that have now been released raise obvious concerns and it is absolutely right and proper that there is a full investigation into this matter, which the Met will fully support."

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Last night the IPCC revealed that a number of the officers caught up in the incident had yet to come forward. It is believed that the footage shows City of London Police dog handlers alongside members of the Metropolitan Police Tactical Support Group.

The IPCC also revealed that it had ordered a second post-mortem examination in an effort to determine if there was any external factor behind Mr Tomlinson's fatal heart attack.

Last night the family issued a statement that thanked the media for, "bringing crucial evidence to light".

Initially the Metropolitan Police spoke of how officers had on 1 April tried in vain to save Mr Tomlinson's life, claiming they had even been pelted with missiles, after he collapsed outside the Royal Exchange Building in the City. Investigators were told that there had been no contact with police before he collapsed.

But the story took on a very different turn when a New York fund manager passed film to The Guardian newspaper, which showed Mr Tomlinson being attacked from behind by an officer in riot gear as he walked away from them with his hands in his pockets.

Deborah Glass, of the IPCC, said the decision to take over the investigation from City of London Police was made after the video came to light. She said: "People are rightly concerned about this tragic death and this footage is clearly disturbing.

"This is a criminal investigation and we will, of course, be discussing it with the Crown Prosecution Service. At the moment the investigation is focused on identifying the officers in the footage. Several have already come forward and all efforts are being made to trace those who haven't."

The attack highlights concerns about police tactics during the demonstrations, during which 5,000 people gathered in the City to protest as the G20 political leaders prepared to meet.

Mr Tomlinson's son, Paul King, 26, said yesterday that the family were distraught. "It is very important for the sake of the family here and his kids, we just want justice.

"Until everything comes out and we have the right evidence, we cannot lay our father to rest."

On the night he died, the 47-year-old, known as Tommo to his friends, was on his way to the Lindsey Hotel, where he had lodged since October. Friends of Mr Tomlinson, a father of nine originally from Matlock, Derbyshire, described him as a man who had lived an itinerant lifestyle. The Home Secretary, Jacqui Smith, called for the IPCC inquiry to be completed "as quickly as possible". The Labour MP David Winnick, a member of the Home Affairs Select Committee, said questions will be asked about "misleading" police statements in the hours after Mr Tomlinson's death.

Police response to Ian Tomlinson's death

*1 April – Statement from the Metropolitan Police

A member of the public went to a police officer on a cordon to say that there was a man who had collapsed round the corner.

The officer sent two police medics through the cordon line where they found a man who had stopped breathing. They called for LAS [London Ambulance Service] support about 19.30. The officers gave him an initial check and cleared his airway before moving him back behind the cordon line to a clear area where they gave 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. LAS took the man to hospital where he was pronounced dead.

*4 April – Statement from the City of London Police

A post-mortem examination ... found he died of natural causes. [He] suffered a sudden heart attack while on his way home from work. The family thanked all the people who rushed to Ian's aid when he collapsed and said how grateful they are for all the efforts made to help.

*8 April – Statement Sir Paul Stephenson, Commissioner of the Metropolitan Police

My thoughts are with Mr Tomlinson's family at this time. The images that have now been released raise obvious concerns and it is absolutely right and proper that there is a full investigation into this matter, which the Met will fully support.