A video, shown in footage on the Guardian website, reveals British police attack on man who died at G20 protest.

Ian Tomlinson, a newspaper seller, who died at last week's G20 protests in London, was attacked from behind and thrown to the ground by a baton-wielding police officer in riot gear, dramatic footage obtained by the ­Guardian shows.

Moments after the assault on ­Tomlinson was captured on video, he ­suffered a heart attack and died.

Police attempted to resuscitate him before he was taken by ambulance to hospital where he was pronounced dead.

"Ian clearly had his arms in his pockets and back towards the police. There is no need for them to step in towards him," Tomlinson's son Paul King was quoted by newspapers as saying.

"(The video) clearly shows that Ian did have an altercation. Now we can say, 'yes he did.' Up until now it has been 'if'. But now we've seen it, we want answers."

Criminal probes

Home Secretary Jacqui Smith said on Wednesday police officers could face criminal probes if an inquiry into events that culminated in the death of a man during G20 demonstrations in London showed action was warranted.

The Liberal Democrats said the incident shown on the video was "sickening" and have called for a full criminal investigation.

Smith agreed there should be action if that was the conclusion of an Independent Police Complaints Commission (IPCC) inquiry.

"I'm glad that the IPCC themselves called for further evidence in order to do that investigation as quickly and thoroughly as possible," she told the BBC.

"If it identifies the need for a criminal investigation, that also needs to be pursued."

The IPCC said it was now analysing the video and other evidence.

Witnesses had told the police watchdog that Tomlinson did have contact with officers before he died, but until now this had not been shown in photos and CCTV pictures obtained of the man making his way home through the protests.

"This video clearly shows an unprovoked attack by a police officer on a passer-by," said the Lib Dems' justice spokesman David Howarth.

"The officer concerned and the other officers shown in the video must immediately come forward."

Tomlinson had not been involved in the protests by some 4,000 people the day before world leaders gathered for a summit to tackle the global economic crisis.

 Agencies