Thousands of anti-war protesters have taken to the streets of London to mark the anniversary of the start of the war in Iraq.
George Galloway told protesters the war had not been legitimate
Two protesters breached security at the Houses of Parliament and scaled Big Ben sparking concerns about policing around the government.
Madrid and New York are among cities around the world where demonstrations are also being held against the war.
Police said some 25,000 people joined London's march to Trafalgar Square.
However, organisers estimated three times that amount of protesters were involved.
The event has been backed by the Stop the War Coalition, CND and Muslim Association of Britain.
One protester, 79-year-old Joe Rossi, from Ashford in Kent, said he did not believe the war had done anything to combat terrorism.
"If anything, it's bringing terrorism closer to our shores," he said.
"I'm fed up with the government deceiving us and acting against the will of the people."
MP George Galloway told the rally in Trafalgar Square the war had not been legitimate.
"Just because they call themselves the international community, that doesn't mean their actions are right."
He urged voters to turn the 10 June European elections into a referendum on Tony Blair's future.
Thousands of balloons were released in memory of those killed in the Iraq conflict, as well as the Madrid bomb attack victims.
Demonstrations have taken place in about a dozen towns around Spain with a large protest due to take place in the capital on Saturday evening.
Tens of thousands of anti-war protesters have marched in Rome with similar events being held throughout the world including Sydney and Tokyo.
Riot police were deployed at rallies in India, Egypt, Turkey and the Philippines.
Police said the two Greenpeace protesters scaled Big Ben at 0615 GMT, unfurling a banner saying "Time for Truth".
Police arrested the two men - named by the group as brothers Harry and Simon Westaway from Lewes in Sussex - and responded to the breach of security by saying they were confident they "would not have been able to access inside the Palace of Westminster itself".
Greenpeace said they did not encounter any security in getting over two fences and climbing the tower.
The group sent another volunteer to the front gate of the site to inform police it was a "peaceful protest and not a terrorist attack".
One of the climbers Harry Westaway said in a statement: "We have achieved what we wanted. It's wet and windy but it's worth it.
"It's about time Tony Blair told us the truth."
The campaigners say they are concerned about the continuing presence of US and other forces in Iraq.
Lindsey German, of the Stop the War Coalition, said: "We are continuing to march because we want to make clear the strength of anti-war feeling in this country.
"Everything we said about the war has turned out to be true and everything the government said has turned out to be a lie.
"The tragic events in Madrid shows clearly that we are not safer from terrorism."
Anti-war campaigners inflated a model of a Trident missile, accompanied by activists dressed up as weapons inspectors.
Last year more than a million people took part in a march in London against the impending war.