An investigation is under way after a police officer was caught on video appearing to punch a demonstrator in the face during a rally in Bolton.
Protester Alan Clough, 63, was accused of using threatening behaviour during a protest against the English Defence League in March.
However, the case was dropped when a video of him allegedly being hit by an officer in riot gear came to light.
Greater Manchester Police (GMP) said it was investigating.
The case against Mr Clough, who had been protesting with Unite Against Fascism (UAF), was due to be heard at Bolton Magistrates' Court on Wednesday.
However it was dropped 24 hours before it got to court after the Crown Prosecution Service (CPS) viewed the video.
A CPS spokesman said: "Having viewed footage received from the defence team, we felt that there was no longer a realistic prospect of securing a conviction.
"Given this, the correct decision was to drop the charge against Mr Clough."
The footage was shot by Granada TV and shows crowds of protesters, police pushing them back, drawing their batons and Mr Clough appearing to get punched in the face by an officer before he is dragged away and arrested.
Police in yellow high-visibility jackets, riot police and mounted officers were involved in the clash, one of many that took place between police and those taking part in the rally.
Justice4bolton, a campaign group which claimed police used heavy-handed tactics on the day, said it had received numerous reports of violent police behaviour during the demonstration.
A spokesman for Greater Manchester Police, said: "GMP has received a complaint in relation to footage which appears to show a man being inappropriately struck during his arrest at the EDL protest in Bolton in March, 2010.
"The footage is now being viewed by officers from Greater Manchester Police's professional standards branch.
"An internal investigation is now under way, as a result it would be inappropriate to comment further at this stage."
Around 2,000 members of the right-wing EDL gathered alongside 1,500 UAF protesters at Victoria Square in Bolton town centre on 20 March.
More than 60 people were arrested and several participants were injured during the rally.
The EDL claims it protests against "militant Islam" and Sharia law but the UAF says the league promotes racism and fascism.
Mr Clough went to the demonstration with four men and three women, aged between 58 and 70, who he knows from anti-fascist and peace movements in Bury.
A spokeswoman for Justice4bolton added: "The emerging body of evidence raises serious questions not only about the acts of individual officers but about the broader approach taken by the police toward those seeking to stand up against racism and fascism."