(Updates with police)

BAGHDAD, March 31 (Reuters) - A mortar barrage hit Baghdad's heavily fortified Green Zone, which houses Iraq's government and the U.S. embassy, police said, a day after Shi'ite cleric Moqtada al-Sadr ordered his fighters to stand down.

The Green Zone has come under intense mortar and rocket attack over the past week as Mehdi Army fighters loyal to Sadr have battled Iraqi and U.S. security forces in the capital and in southern Iraq.

On Sunday Sadr ordered the Mehdi Army to stop fighting Iraqi security forces and to withdraw from the streets. But the cleric has previously acknowledged there are rogue elements within the militia that have disobeyed a truce he first called last year.

A siren wailed inside the U.S.-protected compound in central Baghdad and a recorded voice warned people to take cover amid the sound of explosions, Reuters witnesses said.

A dust storm enveloping the city made it difficult to see where the missiles were landing, but police said a volley of at least six mortars had hit the Green Zone. They had no details of any casualties.

A U.S. embassy spokesman was not immediately available for comment. The embassy has ordered staff in the zone to stay under cover where possible and wear body armour and helmets when in the open. (Reporting by Ross Colvin; Editing by Catherine Evans)