Six people have been arrested as about 100 campaigners staged a protest in Parliament Square against the Serious Organised Crime and Police Act.
Several people have been arrested at the protest
The new act, parts of which came into force last month, bans demonstrations within a half-mile radius of Parliament unless cleared by police.
Unauthorised demonstrators could be removed, arrested or fined £1,000.
Police warned protesters before making the arrests to chants of "Shame on you".
One woman lay on the ground and was dragged off by two policemen.
Earlier officers had been handing out leaflets warning demonstrators that they were in violation of the law.
Anti-war activist Brian Haw is the only person in the country allowed to protest without authorisation as his four-year vigil pre-dates the new law.
The act was brought in to remove Mr Haw's vigil from Parliament Square.
But he won a legal battle to continue protesting outside Parliament at the High Court.
As the arrests were being made he shouted to police: "Officer address your heart, officer why are you here?"
Speaking about the protest on Sunday, he said: "I'm the last of the Mohicans, I'm the last of the Great British.
"My fellow compatriots have been denied a voice. I'm outraged by this, I'm outraged that the police are busy chasing old ladies with peace signs down Whitehall when there are bombs going off in London."
Anyone else wishing to demonstrate within the designated protest zone, must apply to the Metropolitan Police Commissioner at least six days in advance or if not "reasonably practicable" within 24 hours in advance.
The zone, which includes the square, extends as far as the London Eye, and includes all of Whitehall and a large part of Westminster.
On Monday five anti-war activists demonstrating against the ban were arrested.
They were among 50 people who gathered in the Square and wore black gags to symbolise the "infringement" of their right to protest.