As police prepare to face some of the biggest street protests for several years, a committee of MPs has accused them of abusing their powers and being heavy-handed in dealing with demonstrators.
Parliament's Joint Select Committee on Human Rights said police were misusing counter-terrorism laws, anti-social behaviour legislation and the Protection from Harassment Act to deal with protestors.
"The right to protest is a fundamental democratic right and one that the state and police have a duty to protect and facilitate," said the committee's chairman Andrew Dismore.
"Of course, there is a balance to be struck between the rights of protestors, the police and the public (including protest targets) but the state must not impose restrictions unless it is necessary, and proportionate, to do so."
The warning comes as thousands of demonstrators prepare to protest in London at next month's G20 summit.
The committee said police were using stop and search powers to intimidate and conducting wide-ranging seizures of property.
At a Climate Camp protest in Kent last year, police had even seized tent pegs and a clown costume, it said.
The Chief Constable of Kent has now referred the policing of that demonstration, near a power station at Kingsnorth, to the Independent Police Complaints Commission.
Police were monitoring journalists, refusing them access to protests, not recognising press cards and even assaulting them, the National Union of Journalists told the committee.
The MPs said it was unacceptable that journalists had to resort to taking court action against officers "unlawfully" interfering with their work.
The Metropolitan Police Service said human rights and the right to protest were at the heart of its policing philosophy.
"However, the Met also has a duty to ensure that the rights of others are protected," it said.
"We will always facilitate lawful protest and are committed to doing so but do have to minimise the disruption caused to others going about their lawful business."