Police arrested four people on Monday during an anti-war protest which was given the go-ahead only at the eleventh-hour, Scotland Yard said.

The four were detained for offences including assault on police officers and obstruction.

Hundreds of protesters marched from Trafalgar Square to parliament to call for the government to withdraw troops from Iraq as politicians gathered on the first day after the summer recess.

Police, who had earlier threatened to use a Victorian law to prevent the Stop the War Coalition demonstration, said the protesters had broken an agreement.

"It was disappointing that, after assurances from organisers that they had no intention of disrupting parliament or preventing MPs from attending, a group of demonstrators held a 'sit-down protest' in the road in Parliament Square," Deputy Assistant Commissioner Chris Allison said in a statement.

"To my mind this was not about lawful protest but about those who wished to deliberately flout the law and inconvenience others."

Permission to hold the event was given less than an hour before the start, organisers said.

Authorities had effectively banned the march under the Metropolitan Police Act of 1839, which dates from the time of the Chartist protest movement, a period when the ruling class believed they were on the brink of social revolution.

The Stop the War Coalition had been determined to go ahead with the march even without the official green light.

Convener Lindsey German told BBC Radio Four: "We think, in a democracy, we should be allowed to go to our parliament."

She also questioned the timing of the move.

"We have to ask: why is it (the Act) being raised at this time? I guess that Gordon Brown would like to draw a line under the war in Iraq and demonstrations on the first day that parliament comes back are probably an embarrassment to the government."