Organisers said there were 40,000 people
Demonstrators called for troops to be withdrawn from Iraq and Afghanistan and for Gaza's borders to be re-opened.
Organisers said 40,000 people took part in London but police said 10,000. A Stop the War spokesman said Iraq had made the world a more dangerous place.
But a Foreign Office spokesman said Iraq was making "steady progress".
In London there were speeches from the leaders of a range of groups including CND and the Palestinian Solidarity Campaign.
As well as anger over Iraq and Afghanistan, there were also calls for no action to be taken against Iran.
Speaking at the rally in Trafalgar Square, the Liberal Democrat peer Baroness Tongue told the BBC that the war in Iraq had been an illegal act, supported by false documentary evidence.
"We feel that there are people who have literally got away with murder. We have people who have made an illegal war happen, and no-one has brought them to book, and it's about time we did."
Former Labour cabinet minister Tony Benn said: "The troops in Iraq have caused devastation. It's the same in Afghanistan."
'Hidden war'
Green MEP Caroline Lucas called for Tony Blair and Gordon Brown to be prosecuted for war crimes.
And a spokesman for the Stop the War Coalition said: "Estimates suggest as many as one million have died violent deaths as a result of the occupation of Iraq.
Glasgow hosted another demo
"Despite talk of a change of attitude to Bush's wars, Brown is sending more troops to Afghanistan. This hidden war is fast becoming a disaster mirroring Iraq."
Peace campaigner Bianca Jagger said it was "astonishing" that former prime minister Tony Blair and US President George Bush had not been called to account for the Iraq war, which she called an "unmitigated disaster".
Meanwhile in Glasgow hundreds of protestors also marched through the city, waving placards saying "Troops out of Iraq and Afghanistan" and "Don't attack Iran" and "Freedom for Palestine".
A Foreign Office spokesman said Stop The War's description of the situation in Iraq, Afghanistan and Iran was "simply not accurate".
No arrests were made
"In Iraq, there is clear evidence we are making steady progress, particularly in terms of security. We have also acknowledged that mistakes were made, and drawn the appropriate lessons."
In Afghanistan Nato forces are winning the struggle against the Taleban, he said, with 5.4 million children in school compared to an estimated one million children in 2001.
"Iran still has many questions to answer over its nuclear ambitions - we are pursuing UN-approved sanctions to encourage Iran to provide greater transparency," he said.
"And in Pakistan we are encouraged by initial moves to form a new, democratic government which reflects the will of Pakistan's people."
Protesters leading the march 
Yesterday, thousands of people attended anti-war protests in London and Glasgow organised by the Stop the War Coalition. The event marks nearly five years since the 2003 invasion of Iraq which begun the Iraq War. The main themes were "Troops out of Iraq and Afghanistan", "Don’t attack Iran", and "End the siege of Gaza". 

In London, protesters gathered in Trafalgar Square before marching down Whitehall to Parliament Square, across Westminster Bridge, along Lambeth Palace Road before crossing the River Thames again at Lambeth Bridge, returning to Parliament Square via Millbank.

A spokesman for the Foreign Office labelled the Stop the War Coalition's description of the current situation in Iraq as "simply not accurate", saying, "In Iraq, there is clear evidence we are making steady progress, particularly in terms of security. We have also acknowledged that mistakes were made, and drawn the appropriate lessons".

Estimates as to the number of protesters in London range from 10,000 according to the Police, up to 40,000 according to the organisers.