Determined to quash growing protests against Islamic rule, Iranian leaders responded with force against thousands of demonstrators and onlookers late Thursday night, using baton-wielding riot police and plainclothes security forces to beat and shove people into dispersing. Government forces attacked both secular protesters and hundreds of vigilantes sanctioned by Iran's clerical rulers, who arrived on motorbikes to clash with the pro-reform demonstrators. "We have five people to their every one," commented one police officer as he moved the crowd along. "This is not getting out of control tonight." Thursday's street riot _ the third in as many days _ was perhaps the largest violent demonstration against Iran's ruling clerics in four years. The protests come on the heels of the Bush administration's calls for a popular revolt against the country's harsh Islamic rule. Although the clerics around the country's supreme leader, Ayatollah Ali Khamenei, frequently clash with Iran's elected reformist government, both sides united against Thursday's protests. Earlier Thursday, Khamenei accused America of trying to turn disgruntled Iranians into mercenaries. Washington had realized it could not overthrow the Islamic regime militarily and sought to "create a chasm between the regime and the populace," Khamenei charged during a speech in the southern Iranian city of Varamin. Khamenei warned protesters that police would be "pitiless" toward troublemakers, but also urged the vigilantes who support him not to give a "pretext to the enemies" by clashing with demonstrators. How many people were injured or arrested in the melee that ensued in the Iranian capital couldn't be immediately determined, although fresh blood stained the sidewalk outside the dormitory complex of Tehran University where the demonstration started. At 11 p.m., about 150 protesters gathered in front of the complex to coax students into joining them, as they had two nights previously. "Students, students, we are waiting for you. Students, students, we'll protect you," they shouted. "Political prisoners must be freed." Police soon rushed the crowd, which fled. Club-wielding plainclothes officers of the much-feared Information and Security Ministry nabbed three young men, roughly pushed them into a van and drove off. A reporter's notes were confiscated and ripped up. From inside the campus' iron fence, students chanted "Death to the dictator," referring to Khamenei, and pelted police and hard-line thugs with rocks. While police and security forces were successful in dispersing the crowd in front of the university and keeping vigilantes at bay, they could not quell the demonstration elsewhere in the city. Thousands of drivers began honking their horns in unison on streets leading away from the university. Many occupants got out and cheered, erupting into chants of "Political prisoners must be freed" and singing popular pro-Iranian anthems. Residents of nearby apartment buildings launched fireworks and showered torn-up newspaper down on the crowds like confetti. At one hotel along the road, the night guard turned the neon sign on and off to support the protest. As one honking driver put it: "We are happy people finally waking up." Many protesters vowed to continue their activities every night through July 9, the four-year anniversary of student demonstrations at the same dormitory complex that sparked the worst street riots since the 1979 revolution ousted the late shah. The 1999 riots lasted three days and left at least one student dead. (EDITORS: STORY CAN END HERE) The current demonstrations are being spurred by pro-shah Iranian expatriates. On Thursday, via a satellite broadcast from a Los Angeles-based television station, they urged demonstrators to take to the streets and push for an overthrow of the Islamic regime. Iran's rulers are anxious for the demonstrations not to get out of hand, as deaths and injuries of protesters during the shah's time prompted the already disgruntled population to overthrow him. Dissatisfaction with the current government is high, even for popularly elected President Mohammed Khatami. After voting him into office six years ago on a platform of reform, Iranians _ especially younger ones _ are fed up with the slow pace of political and social change. ___ (c) 2003, Knight Ridder/Tribune Information Services.