Boris Johnson has been accused of using Parliament Square as his “private back garden” in an attempt to crush the fledgling Occupy Democracy movement.

Lawyers have written to London’s mayor threatening legal action after he sanctioned the construction of “unlawful” fences around the square, which campaigners claim are a deliberate attempt to stop them protesting peacefully. Parliament Square is considered to be one of the most important sites in the country for demonstrators and is maintained by Johnson’s Greater London Authority (GLA). Protesters argue that the square was conceived as a place for public meetings, focusing particularly on issues that they believe are being ignored by MPs.

However, four days into a planned 10-day protest last October by Occupy Democracy, a new incarnation of Occupy London, large fences appeared on the square, squeezing protesters on to a narrow strip of grass away from parliament. Lawyers for the human rights group Liberty have written to Johnson warning that if he does not accept the fence is illegal, the GLA faces a judicial review.

Rosie Brighouse, Liberty’s legal officer, said: “Peaceful protest in the UK has a long, proud history. Many of the freedoms we enjoy were won because people were prepared to demonstrate. Parliament Square is not the mayor’s private back garden. He has a duty to facilitate this political activity, not prevent it – especially in the very heart of the world’s oldest democracy.”

The GLA says that the Occupy demonstrations were unauthorised because protesters did not try to contact City Hall, although legal papers indicate that Scotland Yard knew of the protests in advance.

Meanwhile, a freedom of information request has revealed that £250,000 was spent on policing the October protests, with more than 500 officers deployed on one day. At one point witnesses counted 140 officers trying to confiscate a piece of tarpaulin that 30 people were sitting on, compounding claims that the Met’s response was heavy-handed.

The legal threat is issued on behalf of a social justice campaigner, George Barda, 38, who argues that his right to protest was compromised by the fence. He said: “The 1% have conspired to crush a crucial and legitimate democratic movement because they know it resonates with the majority of people.

“The judicial review will be a litmus test for whether the courts are also captured by these same powerful interests or whether they do the right thing and hold accountable the GLA and protect our right to peaceful assembly.”

A letter from lawyers to City Hall states: “It is suspected that the decision to erect the fencing was made for the sole or primary purpose of preventing Occupy Democracy from holding any demonstration on PSG [Parliament Square Garden]. The GLA say the fence was erected to protect the square’s grass.”

A subsequent Occupy protest in November was even more constrained after the fenced-off area had been expanded. “The demonstrators were unable to enter PSG at all and the only area they could assemble in was the small, clearly unsuitable area in front of the Supreme Court, which is the opposite side of the Square from Parliament and is of limited visibility to passers-by,” says the letter from Liberty.

Tensions between Johnson and Occupy campaigners are nothing new. The Tory mayor infuriated Occupy London protesters camped outside St Paul’s cathedral in 2011 after he described them as “crusties” engaged in a “thoroughly maddening protest against capitalism”.

Occupy Democracy also claims the Parliament Square fencing was erected in effect to block the group from growing because it has prevented people from holding and attending planned seminars and workshops which were to be sited alongside a planned temporary library, kitchen and toilets.

The group plans to return to Parliament Square for one weekend each month until the general election in May 2015, arguing that the fence poses a “significant risk to the viability of this plan”.

A GLA spokesman said: “Occupy Democracy has since October stated their intention through social media to occupy Parliament Square Garden on three occasions – each of which has been unauthorised, with no permission from the GLA sought or granted. Fencing has been installed to prevent an illegal encampment and to protect the square’s assets, including the grass and the statues.”

The spokesman added: “The mayor supports the right to peaceful protest, but it must be done within the law. There are bylaws in place affecting the square and any activity, whether a protest or other event, requires the prior written permission of the GLA before it can proceed.”