Witnesses said police fired tear gas and water cannons at thousands of protesters who rallied in Teheran Saturday in open defiance of Iran's clerical government, sharply escalating the most serious internal conflict since the 1979 Islamic Revolution. Eyewitnesses described fierce clashes near Revolution Square in central Teheran after some 3,000 protesters chanted "Death to the dictator!" and "Death to dictatorship!" Police responded with tear gas and water cannons, the witnesses said. English-language state TV said a blast at the Teheran shrine of Ayatollah Ruhollah Khomeini had killed one person and wounded two, but the report could not be independently confirmed due to government restrictions on independent reporting. Supreme Leader Ayatollah Ali Khamenei warned opposition leaders on Friday to end street protests or be held responsible for any "bloodshed and chaos" to come. Eyewitnesses contacted by The Associated Press said thousands of police and plainclothes militia members filled the streets Saturday to prevent rallies. Fire trucks took up positions in Revolution Square and riot police surrounded Teheran University, the site of recent clashes between protesters and security forces, one witness said. Web sites run by supporters of opposition leader Mir Hossein Mousavi said he planned to post a message, but there was no statement by the time of the planned street protests at 4 p.m. (7:30 a.m. EDT, 1130 GMT). Some pro-reform Web sites called for people to take to the streets Teheran Province Police Chief Ahmad Reza Radan said earlier in the day "police forces will crack down on any gathering or protest rally which are being planned by some people." English-language state TV said the country's highest national security body had ordered security forces to deal with the situation. It did not elaborate. The government statements were the most explicit warnings yet of force against protesters who gathered in massive rallies last week to demand the government cancel and rerun elections that ended with a declaration of overwhelming victory for hardline President Mahmoud Ahmadinejad. Mousavi says he won but Ahmadinejad stole the election through widespread fraud. Khamenei sided firmly with Ahmadinejad Friday, saying the result reflected popular will and ordering opposition leaders to end street protests or face the consequences. The statement effectively closed the door to Mousavi's demand for a new election, ratcheting up the possibility of a bloody confrontation.