An investigation has begun into whether three Metropolitan Police officers colluded to falsely arrest a man at a tuition fees protest in central London.
A 20-year-old breached a cordon on 9 December and was chased and caught by an officer, the Independent Police Complaints Commission (IPCC) said.
But a subsequent conversation between the arresting officer and colleagues has sparked an inquiry, the IPCC added.
It will also consider when and how the man suffered a chipped tooth.
The matter was referred to the IPCC by the Metropolitan Police, a Scotland Yard spokesman said, and the force awaited the inquiry's findings.
One of the officers had been wearing a sound device which recorded the pursuit and the man's detention in Parliament Square.
It also recorded a discussion between the group of police and will now be examined as evidence.
"We are investigating a serious allegation that an officer colluded with colleagues to abuse his position by arresting a young man on false grounds," said the IPCC's commissioner Rachel Cerfontyne.
"We will also be looking at the circumstances of how the man suffered a broken tooth during his detention."
The demonstration on 9 December was one of a series of rallies opposing the increase in tuition fees in England.
It was the day of the attack on a limousine carrying the Prince of Wales and the Duchess of Cornwall, which was surrounded by a mob as the couple were driven to the London Palladium.