Ian Tomlinson, 47, died last week during the Group of 20 summit, which was accompanied by large protests, after suffering a heart attack.
Tomlinson was not a protester, but rather a newspaper seller trying to get home from work to watch a soccer match. Police told reporters that Tomlinson did not have any contact with police before his death.
The amateur video, obtained by British daily The Guardian, clearly disproves that information. It shows Tomlinson walking near the Royal Exchange, with his hands in his pockets, followed by a line of officers, some with dogs and in riot gear, urging Tomlinson forward.
The man keeps walking and does not address the officers or offer resistance; one officer then appears to strike Tomlinson from behind with a stick and push him in the back, sending him flying to the ground.
The film was shot around 7:20 p.m., only a few minutes before Tomlinson collapsed and died of a heart attack, by a fund manager from New York who was in the city on business, the newspaper said.
The Independent Police Complaints Commission, the British police-monitoring agency, launched an investigation Wednesday after The Guardian provided evidence related to Tomlinson's death, containing the video, photos and witness statements.
The video has sparked a public outcry in Britain, with politicians and the public calling for a thorough investigation into the death that came as thousands of protesters faced off with police during the G20 summit on April 1. The protests ended without serious injuries, but Tomlinson's death nevertheless managed to cloud police work related to the summit.
Scotland Yard has since held several top-level crisis meetings, and the IPCC will question several officers in relation to the assault -- including the one who hit and pushed Tomlinson. He was part of a group of officers who turned themselves in after the video was published.
The officer could face manslaughter charges if a second autopsy authorities ordered reveals the assault led to Tomlinson's death.
Police have faced off with protesters all over Europe during the past few days. Roughly 25,000 police were dispatched to secure a NATO summit last weekend in Strasbourg, France, and the two German cities of Kehl and Baden-Baden.
Anti-NATO demonstrations remained peaceful in Germany, but on the French side, black-clad protesters clashed with police several times, and authorities used tear gas to disperse protesters trying to march into Strasbourg's city center.
On April 4 black-clad protesters in Strasbourg torched a hotel and a customs facility near the French-German border. However, organizers of the anti-NATO protest marches have said police may have secretly had their people with the worst rioters.
A video sent to reporters shows French police dressed in riot gear throwing stones at protesters.
It remains unclear whether the protesters had attacked police before filming started, but as police start throwing, a man using a megaphone from inside the demonstration urges police "to keep calm, so that this protest march can pass calmly and peacefully, as has happened until right now, so stop the provocations."
Berlin, a city that for years has seen violent protests involving far-left rioters on May 1, is already gearing up for the worst violence in years.
Several cars have been torched recently, and police are expecting more violence leading up to May Day.