Police erected a 2 metre metal fence around much of Parliament Square on Tuesday and arrested 15 demonstrators including Green party peer Jenny Jones.

Lady Jones, 64, chairwoman of the London assembly’s economy committee and deputy chair of its police and crime committee, was arrested for “obstructing police” as they moved in to break up the demonstration. She was “de-arrested” after giving details suitable for a summons.The politician could now face prosecution, Scotland Yard said.

“The evidence in this case will be considered and a decision made whether to proceed with a prosecution,” a Met spokesperson said.

Jones had been to see what was happening at the Occupy London protest on Tuesday morning after her office had been contacted by protesters complaining about heavy-handed police tactics.

The Greater London Authority said the fence was erected to enable the grass to be reseeded. Protestors insist it is their right to use the square to campaign on issues such as the privatisation of the NHS, tuition fees, the bedroom tax and benefit cuts for people with disabilities.

“They shouldn’t have arrested anybody here,” said Jones. “They should have just listened and left them alone.”

According to its website, the goal of the Occupy Democracy campaign is to “direct the energy from current single-issue struggles into a critical mass that can radically challenge the corrupt and unrepresentative system”.

A GLA spokesman said: “The fencing is in place to uphold the byelaws affecting Parliament Square and the Police Reform and Social Responsibility Act. The protesters refused to comply with instructions to keep off the grass and enable workmen to get on with vital repairs and maintenance that are needed as a result of their actions.”

Jones is scheduled on Wednesday to give an address to the protesters on policing and civil liberties. She went to the protest on Tuesday after hearing that the police had arrived in as many as eight vans and their response to the protest was “disproportionate”.

Speaking after she was released, Jones said: “The police have a duty to facilitate peaceful protest in this country which people have a legal right to do, but that appears to end as soon as you come within shouting distance of the Westminster village. The people who run this country should not be able to tuck demonstrators away out of sight. Parliament needs to listen and people should have the right to get their voices heard.”

The swoop by police follows a crackdown on Sunday night when large numbers of officers converged on the square to enforce a “desist notice”.

Scotland Yard said in a statement on Tuesday: “This morning there were around 15 people demonstrating in Parliament Square. The group on the grassed area had been sleeping on a tarpaulin which is prohibited under the Police Reform and Social Responsibility Act of 2011, namely they have items with them to enable them to sleep in Parliament Square.

“We informed them of the breach of legislation and asked them to leave, this was done on a one-to-one basis. They were told that if they failed to leave they would be removed from the site and would be liable to summons for the offence.

“The tarpaulin has been removed and 15 people have been arrested for failing to provide details suitable for a summons. They have been taken to a central London police station. Our role is to facilitate peaceful protest whilst balancing the needs of London communities and minimising any disruption. We will not take sides, but we seek to apply the law fairly and proportionately.”