The Liberal Democrats are demanding a criminal inquiry after video footage of the G20 protest showed a police officer pushing over a man who later died.
Newspaper vendor Ian Tomlinson, 47, suffered a heart attack shortly after the incident, outside the Bank of England in central London last week.
Lib Dem justice spokesman David Howarth said the footage showed a "sickening and unprovoked attack" by police.
Mr Tomlinson, who was not protesting, had been making his way home from work.
The pictures, shot at 1929 BST (1829 GMT) at Royal Exchange Passage, initially show him walking away from a group of police officers.
He then receives a two-handed push from an officer, landing heavily before remonstrating with the police.
Mr Tomlinson collapsed and died of a heart attack after walking to nearby Cornhill, where he received first aid from police.
Whether or not they can prove the police officer actually caused his death, I think this will be the end of the policeman's career.