Anti-war campaigners are preparing what they expect to be a massive march marking a year since the UK took part in the Iraq conflict.
Last year hundreds of thousands marched against the war
The Stop the War Coalition plans come as a BBC poll suggests 48% of Britons now think military action was right.
The protesters said it was "urgent" people showed their opposition to the war and held the government to account because of its "lies".
Similar protests will be held in cities across the world, including Madrid.
Protesters expect a huge turnout in the Spanish capital following last week's bomb attacks that killed 201 people and injured more than 1,400.
A poll carried out by ICM for the BBC's Newsnight programme questioned more than 1,000 people between 10 and 12 March.
A total of 40% said they felt the UK Government had exaggerated but not lied about the threat of weapons of mass destruction in the run-up to the Iraq war.
The poll also suggests 43% of people remain opposed to the conflict, with 9% undecided.
At Saturday's march a black balloon will be released in memory of civilians and soldiers killed in Iraq, and a minute's silence will be held in memory of the Madrid attack victims.
Last year hundreds of thousands of people took part in a march in London against the impending war.
One of the organisers, and chairwoman of CND, Kate Hudson, said the march would make clear the strength of anti-war feeling that still existed.
"More and more people are convinced by our argument. The government is refusing to release information to the public about why we went to war.
"Questions such as 'Why did we go into an illegal war?' and 'On what evidence?' are not going to go away. Our government sinks further into disrepute by avoiding them."
Ms Hudson insisted the BBC poll did not indicate growing support for the war and subsequent occupation of Iraq.
"Even though Tony Blair thinks he did the right thing, the majority of the British population don't agree with him."
One Year On Iraq - A Newsnight Special is screened on Tuesday at 2100 GMT on BBC Two.