The new law is aimed at unauthorised protests in Westminster, such as Brian Haw's 4-year anti-war vigil which MPs have complained is an eyesore.

But it will also cover St James's Park, much of the South Bank and Charing Cross to Lambeth Bridge from 1 August.

Critics say the zone goes much further than was originally intended.

Trafalgar Square, the traditional venue for protests, is not included in the "exclusion zone".

Mr Haw has fought off successive attempts to evict him

But Parliament Square has been the scene of several high-profile protests from the "guerrilla gardening" event on May Day 2000 to clashes between police and pro-hunt campaigners last September.

Through it all Mr Haw has been sitting surrounded by anti-war banners, occasionally using a loudspeaker to attack the government's policy in Iraq.

He only leaves to attend court for the as-yet unsuccessful attempts to evict him.

Some MPs say his protest is an eyesore and his loudspeaker distracts them from their work. There are also concerns his piles of placards pose a security risk - although they are regularly searched.

But unauthorised protests by one person are outlawed under powers granted to Home Secretary Charles Clarke in the Serious and Organised Crime and Police Act, passed this year.

What possible justification can there be for banning spontaneous demonstrations across such a wide area? Mark Oaten Lib Dems

Within the zone, police have powers to set time limits on protests and ban loudspeakers and placards.

Police will have to authorise demonstrations and anyone holding a spontaneous protest can be arrested.

The government says there must be a balance between the rights of protesters and those of the people working in the Houses of Parliament.

But Mr Clarke's political rivals say the zone goes too far.

Lib Dem Home Affairs spokesman Mark Oaten said: "What possible justification can there be for banning spontaneous demonstrations across such a wide area?"