By the BBC's Gillian Ni Cheallaigh
In Hyde Park, London
Despite wet and windy March weather tens of thousands gathered in Hyde Park for Saturday's demonstration.
They arrived in coaches early in the morning from all over the country to assemble at midday.
Groups of all kinds joined together for the protest march
The Stop The War Coalition who organised the march said they expected 100,000 or more based on the number of coaches.
Police estimates were lower. On the ground it felt like a very big march.
It set off at 1300 GMT - an hour later people were still waiting to march.
They marched under the Stop The War Coalition banner, but represented were dozens of disparate groups: The Campaign For Nuclear Disarmament, The Muslim Association of Britain, Greenpeace, Socialist Workers and Anti-War youth movements.
They were out to mark the one year anniversary of the war in Iraq and to remind the government of their opposition to the British presence in the country.
While a recent poll suggests people in Iraq want the coalition troops to stay to ensure security, the protesters wanted Tony Blair to bring the British troops home.
It was a colourful and noisy display of anti-war feeling - some banged drums, others chanted slogans.
"US, UK, how many kids have you killed today?" some shouted.
The crowd is marching from Hyde Park to Trafalgar Square for a peace rally with speakers, including George Galloway, Jeremy Corbyn and Lindsey German from Stop The War and Dr Azzam Tamimi from the Muslim Association of Great Britain.
They will be telling protesters here to use their votes in the June European and May local elections to make their anti-war message have political impact and following the example of the Spanish election upset.