Occupy movement protesters descended on central London in an attempt to set up camp outside Parliament.
The pro-democracy group called on supporters to “return to Parliament Square” from 6pm this evening until late on Sunday.
Protesters are banned from setting up camp in front of the Palace of Westminster and Scotland Yard has warned that “an appropriate and proportionate police plan” is in place for this evening’s event.
Dozens of officers were stationed in front of fencing placed around Parliament Square which prevented people from accessing the site.
A spokesman for the Metropolitan Police said it attempted to make contact with organisers but the group had "failed to engage" with the police force.
The force confirmed a Section 60 AA order was in place in the area around Parliament Square which gives police powers to force people to remove masks where they anticipate criminal activity.
The order is in place until 2pm tomorrow, a police spokesman said.
Protester Tom Kay, 21, from Sheffield, branded the police guard around Parliament Square "a disgrace".
He said: "Protest is legal in this country and I think we should be able to protest where we want.
"The police are unwilling to let people protest seriously."
Announcing the event on the Occupy website, a statement said: "We're coming together because we want genuine democracy - free from corporate influence - where our voices count.
"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.
Student protests
Under the Police Reform and Social Responsibility Act, the police can forcibly remove any protesters that decide to set up camp in Parliament Square.
The legislation was introduced after anti-war campaigners spent years occupying the London landmark outside Parliament.
Thousands of students gathered in central London to protest against tuition fees and debt on Wednesday.
The National Campaign Against Fees and Cuts said its winter campaign would be the biggest “fightback” from students since the 2010 demo which culminated in an attack on the Conservatives’ Millbank headquarters.
