London  - British politicians and civil rights groups Wednesday called for a criminal inquiry into how a 47-year-old man died from a heart attack on the fringes of anti-Group of 20 (G20) demonstrations in London after a police encounter last week.

Video footage published Wednesday on the website of the Guardian newspaper showed Ian Tomlinson walking with his back to a group of police officers, hands in his pockets, when a helmeted officer lunges at him from behind.

Tomlinson, who was not protesting, is seen falling heavily to the ground and complaining to the officers before being helped to his feet by passers-by on April 1, the eve of the G20 summit.

Tomlinson, a newspaper vendor, walked on, but collapsed with a heart attack shortly afterwards near the Bank of England, where the demonstrations had taken place and protestors were still milling around.

The Independent Police Complaints Commission (IPCC) watchdog is investigating the incident, but the opposition Liberal Democrats Wednesday demanded a criminal inquiry.

"There must be a full-scale criminal investigation. The officer concerned and the other officers shown in the video must immediately come forward," said the party's justice spokesman, David Howarth, Wednesday.

He said the footage, recorded by a New York fund manager in London's financial district showed a "sickening and unprovoked attack" by police.

The family of the dead man also asked for "answers" and called for witnesses of the incident, stressing that Tomlinson, a keen football fan, would never have taken part in any political demonstration.

Scotland Yard has said it would "not be appropriate" to comment while the IPCC investigation was continuing.