Organisers of the demonstration, taking place on the eve of the Labour Party annual conference, hope to exploit the strong anti-war feelings of two years ago when a million people descended on the capital in an eleventh-hour bid to prevent a conflict.
The fathers of two British soldiers killed in the Iraq conflict will address the protesters before the march starts. Reg Keys, whose son Tom was one of six Redcaps killed two years ago, and Peter Brierley, whose son Shaun died in Kuwait in 2003, will call for British troops to be brought home.
After the march has left Parliament, Sue Smith from Tamworth in Staffordshire, whose son Phillip Hewett died in July when a bomb exploded under his vehicle near Basra, will deliver a letter to the Prime Minister urging him to pull British troops out of Iraq. "I am sitting writing this letter hoping that you will understand how we feel, but I know that you don't," she wrote.
The Stop The War Coalition said the recent clashes involving British troops in Basra highlighted the urgent need to bring the "occupation" to an end.
The demonstration, the twelfth to be held by the coalition over the past few years, will begin outside the Houses of Parliament, where protests have been banned under new laws. The coalition said it wanted to challenge the "stupidity" of the legislation as well as show that British people remained opposed to this country's continued involvement in Iraq.
The convenor, Lindsey German, said: "The Lib Dems have this week called for our withdrawal, Anglican bishops have added their voice to this demand and the Stop the War Coalition has received support for this demonstration from musicians, artists, academics, theatre and film personalities, business, religious and trade union leaders.
"Enough is enough. It is now time, once again, for the British people to step forward into the streets and insist that this time we will not be ignored."
Tony Benn, the veteran peace activist, said most people in the UK were now against Britain's involvement in Iraq and believed that the war was illegal. "This is no longer a minority view and the purpose of the march is to encourage other people to give us support," he said.
Kate Hudson, chairwoman of CND, said: "This will be a very important event on the eve of Labour's annual conference because we want to send a strong message about the sort of foreign policies the party should be pursuing."
The Muslim Association of Britain, which is helping to organise the protest, said it was time to end the "senseless" loss of life in Iraq. "The events in Basra earlier this week prove the most pressing need to withdraw our troops from Iraq. The coalition forces are seen as an occupying force and their continued presence in the region exacerbates the instability in Iraq," a spokesman said.
Ms Smith's protest letter to Tony Blair reads: "Dear Prime Minister, You don't know what it's like to be told your son has been murdered in Iraq and then find out through TV news he wasn't killed instantly, but lay dying at the side of road.
You don't know what it's like to have to meet an aeroplane carrying a coffin containing his body. I can't find the words to describe how that feels but I hope to God, you and your wife never experience it. If Phillip had died for Queen and country, fighting for something worthwhile with the equipment he needed to do his job, I would be proud to say he gave his life fighting for a good cause.
"Instead, I am sitting here with my family trying to contain the tears and the anger that is building up inside me.
"As far as we are concerned my son died for nothing. You effectively sentenced him to death because he was driving a Land Rover instead of a Warrior.
"You were more interested in looking after the interests of the Iraqis than that of your own people. You've let down those three young men and the other 89 British soldiers who have so far died in Iraq and it's time you started listening to me and the other families who want to see our boys pulled out of a hopeless situation."