KERBALA, Iraq, Aug 27 (Reuters) - Clashes erupted on Monday in Iraq's Shi'ite city of Kerbala where tens of thousands of pilgrims have gathered for one of the holiest days on the Shi'ite Muslim calendar, a Reuters photographer said. The photographer said it was not clear who was shooting at whom, but he had seen at least one body lying in the street near his hotel. He said a group of men had tried to storm his hotel to seize cameras from journalists inside. Iraqi security forces have launched a massive security operation in the city to protect pilgrims, deploying thousands of police and soldiers.