That footage was taken by a 38-year-old investment manager from New York, who said he had attended the protests out of curiosity.

A freelance photographer, Anna Branthwaite, told the newspaper that she had witnessed Mr. Tomlinson being attacked with no provocation. She said that after rushing him and pushing him to the ground, a police officer “hit him twice with a baton” as he lay there and then “picked him up from the back, continued to walk or charge with him, and threw him.” He staggered down the street and then collapsed soon afterwards.

After a barrage of complaints and the emergence of the new evidence, the police department said late on Thursday that it had suspended the officer in question, whose face was obscured by a balaclava in the video and whose name has not been released. Officials are also holding an inquest into Mr. Tomlinson’s death.

The Independent Police Complaint Commission, which originally said it would oversee a police investigation of the incident, changed its mind earlier this week, saying it would conduct the investigation itself. It also said that, armed with the new evidence, it had widened its inquiry to “investigate the alleged assault by police on Ian Tomlinson shortly before his death” and to “look into whether that contact may have contributed to his death.”

Image Assistant Commissioner Bob Quick of the Metropolitan Police Department was photographed arriving at No 10 Downing Street on Wednesday carrying a document that outlined details of a major antiterrorism operation. Credit... Steve Back/European Pressphoto Agency

The incident undermines confidence in accountability at the police department, whose image has never fully recovered from the death, in 2005, of Jean Charles de Menezes. Mr. de Menezes, a Brazilian electrician, was fatally shot by the police on the subway during a period of high tension after a series of suicide bombs on London’s transportation system killed 52 commuters. The police originally claimed that Mr. de Menezes had refused to stop when challenged, behaved strangely and led them on a chase through the subway system  all of which later proved false.

The police explained that they had confused Mr. de Menezes with a potential terrorist suspect who lived in the same apartment building as him.