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BAGHDAD, April 8 (Reuters) - Iraqi authorities have imposed a one-day curfew on Baghdad for Wednesday, an official in the Baghdad operations command for Iraqi security forces said.

The official said cars and motorcycles would be banned from the streets of the Iraqi capital between 5:00 a.m. and midnight (0200-2100 GMT) on Wednesday.

Wednesday is the fifth anniversary of the fall of Baghdad to U.S. forces, and followers of Shi'ite cleric Moqtada al-Sadr had announced mass anti-American demonstrations. But Sadr called off the planned protest earlier on Tuesday for security reasons.

Sadr's Mehdi Army militia has been clashing with U.S. and Iraqi forces in Sadr City, a vast eastern Baghdad slum, for the past three days after fighting late last month in Baghdad and the Shi'ite south.