Demonstrators were gathering for the start of the Labour conference in Brighton to protest against Mr Tony Blair's support of President George Bush's war on terrorism.
Up to 10,000 people are expected at the protest, co-sponsored by the Green Party and the anti-capitalist group, Globalise Resistance.
It was originally planned to oppose what organisers called New Labour's "adoption of Tory policies".
But "since Tony Blair threw his weight behind George Bush's military crusade, there is expected to be a significant element of opposition to the escalation of the current crisis", the Green Party says.
Banners attacking British government plans to privatise services within the NHS were strung up from trees at a park in the centre of the city from where the march will start.
Police presence was low on the ground despite 800 officers and a number of Metropolitan police horse patrols on standby for the event.
Sussex Police fear the protest could turn violent and warned they would deal "robustly" with anyone threatening public order.
Officers will be equipped for the first time with "Captor Spray", a fiery pepper spray which temporarily blinds when squirted in the face.
PA