Massive security measures are in place to protect the Labour Party conference from the dual threats of international terrorism and anarchist demonstrators.

In a police operation codenamed Operation Otter a security cordon was thrown around the Brighton Centre, where the conference opens on Sunday.

A five-mile air exclusion zone has been set up over Brighton and armed police and bomb disposal squads are on duty in the city.

Strict security is also in place in and around the hotels where Labour MPs and conference delegates will be staying, including The Grand, which was devastated by an IRA bomb aimed at Margaret Thatcher and her Cabinet in 1984.

Hundreds of metres of steel fencing along Brighton's streets have been erected while armed police began checking all visitors entering and leaving the Brighton Centre.

Protective metal fencing has also been put up around public monuments and statues.

Up to 10,000 people are expected to march on the conference centre on Sunday to protest about issues such as Government plans for privatisation within the NHS.

The Green Party, who are supporting the march, organised by anti-globalisation group Globalise Resistance, said part of the demonstration would also be aimed at President George Bush's declared war on terrorism.

Police also fear hard-line anarchist groups may also use the peaceful protest as an excuse to launch violent clashes, as witnessed during the G8 Summit in Genoa in which one demonstrator was killed.

But today senior police officers said the highest perceived threat to the conference remained from Irish terrorist groups and not those who led the recent terror attacks on America.

For the first time at the conference, armed police have been stationed at entry points to the Brighton Centre, which has been declared a "secure island site" by police.

As witnessed at last year's Labour conference, the centre has been surrounded by a ring of steel and linked to the neighbouring Metropole Hotel by a bridge, allowing delegates to pass freely without being constantly searched.

Other new measures include motorway-style Armco crash barriers that have been installed on the main A259 coast road that runs past the Brighton Centre, designed to keep protesters away from the building.

Security measures have caused traffic delays in the city centre as cars entering central car parks are searched. Police checks had also been set up on main roads leading into the city, where vehicles were being searched at random.

Twenty five police units, each consisting of 25 officers, have been put on standby for the anti-capitalism demo.

Protesters will congregate at The Level at 2pm and march through the Old Steine before converging on the conference.