Occupy London activists defied police warnings and gathered in central London on Friday to set up camp outside parliament.

Demonstrators converged on Parliament Square despite being told by Scotland Yard that they are banned from putting up tents or sleeping overnight by the landmark.

About 100 demonstrators formed a blockade in the road around the square, unveiling banners reading “real democracy now” and chanting “the police should be helping us”.

Long tailbacks formed along Whitehall as motorists sounded their horns, while scuffles broke out between protesters and police as the demonstration moved towards Downing Street.

Author Donnachadh McCarthy, 55, said: “It’s outrageous that in Parliament Square free speech is being suppressed by Boris Johnson’s officers. If you don’t have free speech in front of parliament, you don’t have free speech.”

After marching back down Whitehall toward the square, protesters were met by a police blockade and gathered near the Nelson Mandela statue, playing Free Nelson Mandela by The Special AKA. Nearby, around 80 activists gathered outside the supreme court.

Occupy activists hold a demonstration outside the supreme court. Photograph: Anthony Devlin/PA

John Sinha, one of the organisers of the Occupy protest, said he believed the police blockade of Parliament Square was illegal.

“I suspect they probably have no legality in enforcing it,” he said.

Organisers called for supporters to return to Parliament Square from 6pm on Friday until late on Sunday.

Scotland Yard insisted it had “an appropriate and proportionate police plan in place” for the event, adding it had attempted to make contact with organisers but the group had “failed to engage” with the police force.

A Section 60 AA order was put in place by the Metropolitan police in the area around Parliament Square, which gives police powers to force people to remove masks when they anticipate criminal activity.

Announcing the event on the Occupy website, a statement said: “Our votes, so hard won by the struggles of previous generations, have little value if politicians ignore the population they’re supposed to serve.”

There were a series of clashes with police on Parliament Square last month as supporters of the Occupy movement stood their ground for nine days. Green party politician Jenny Jones was among a group of arrested protesters.