MADRID, March 17 (Reuters) - Tens of thousands of anti-war demonstrators marched in Spanish cities on Saturday in what organizers said were Europe's biggest protests to mark four years of conflict in Iraq.

The largest demonstration was in Madrid where organizers estimated around 400,000 protestors, though city authorities put the total in the tens of thousands. It was one of around 100 anti-war demonstrations across Spain involving cities including Barcelona, Valencia and Seville.

Left-wing political leaders and Spanish celebrities like film director Pedro Almodovar marched in Madrid behind a banner reading "End the occupation in Iraq, shut down Guantanamo".

Protestors walked to a memorial for the 191 people killed in March 11, 2004 train bombings by Islamist radicals.

The bombers said they targeted Madrid because of then Prime Minister Jose Maria Aznar's move to send Spanish troops to Iraq in support of U.S. President George W. Bush.

Aznar was voted out of power days after the attacks, along with his conservative Popular Party.

Spain's incoming socialist government, led by Prime Minister Jose Luis Rodriguez Zapatero, pulled Spanish troops out of Iraq. Zapatero has refused to send additional Spanish troops to Afghanistan, despite demands from NATO members.

Banners at the Madrid march read "Zapatero you're not alone", "Troops out of Afghanistan and "Popular Party, fascists".

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