The Metropolitan Police has paid out ?85,000 in damages to five protesters after admitting officers assaulted and falsely imprisoned them outside the Mexican Embassy.
The protesters were awarded the money following an incident during a demonstration in 2006, in which they were arrested and detained but later acquitted in court.
Their lawyer warned if similar cases are now brought in the wake of their G20 protests in London, the Met could be facing a damages bill running into hundreds of thousands of pounds.
The High Court has approved the settlement and, with costs, the final bill is expected to be more than ?100,000.
Tony Murphy, of Bhatt Murphy, told Channel 4 News: "This case concerned five protesters, the G20 involved hundreds if not thousands of protesters? a large number of whom, that we are seeing, who had their freedoms interfered with, and so you are talking in hundreds of thousands (of pounds)."
In extracts from a letter of apology to the protesters, a Met Detective Chief Inspector wrote: "Firstly, it is accepted that your arrest was unlawful and that any force used on you during your arrest was therefore an assault and battery.
"I am in no doubt about the significant effect that this matter has had on you and on your democratic right to peaceful protest.
"You should not have been arrested and I again apologise for this."
One of the protesters, Agata, told the programme: "This is a democratic country, supposedly, and people have a right to protest and for me it appears that the Metropolitan Police has some sort of policy that prevents people from protesting even if it is perfectly legal and peaceful."