Police arrested four people during the Stop the War Campaign march calling from the withdrawal of troops from Iraq, - after the protest was given the go-ahead by the authorities following outrage at its banning.
Scotland Yard said that the four were detained for offences including assault on police officers and obstruction. Protestors will contest these claims.
Many hundreds of demonstrators marched from Trafalgar Square to Parliament Square to call on the government and Prime Minister Gordon Brown to withdraw troops from Iraq.
The march was timed to coincide with politicians gathereing on the first day of parliament after the summer recess.
Police, who had earlier threatened to use an archaic 1839 anti-Chartist law to prevent the Stop the War Coalition demonstration, said the protesters had broken an agreement.
"To my mind this was not about lawful protest but about those who wished to deliberately flout the law and inconvenience others,"Deputy Assistant Commissioner Chris Allison said in a statement.
The comment was described as "nonsensical and misleading" by protestors, and organisers pointed out that permission to hold the event was given less than an hour before it was due to start, and only after a public outcry.
They say the police action brought about the conflict, and led some people to wish to make a further protest. "But this was only a handful - the vast majority marched peacefully as they had promised all along", an observer told Ekklesia.
The authorities had effectively banned the march under the Metropolitan Police Act of 1839, a period when the ruling classes 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. Veteran MP Tony Benn wrote to the Home Secretary to say that he would not be deterred.
STWC convener Lindsey German told BBC Radio 4 yesterday morning: "We think, in a democracy, we should be allowed to go to our parliament."
She questioned the timing of the authorities' action: "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."
"If the government's threatened ban against the anti-war protest had gone ahead, it would have been an attack on all our liberties", said Coalition chair Andrew Murray.