Muhammad Reza Khatami, the Islamic Iran Participtation Front (IIPF) leader, issued the warning during a party congress on Thursday.
The congress is taking place amid signs that voter frustration could deal the embattled reformist camp a serious defeat in upcoming elections.
"Reformists are trying to prevent the present social movement from being transformed into a violent political revolt or into a scenario of an overthrow (of the government) from outside the country," he said.
"Reforms will continue and deepen in a movement that is calm and progressive."
"We are confronted by two visions of the Islamic republic," Muhammad Reza, the brother of the Iranian president, said.
He was referring to conservative-run courts and legislative oversight bodies who favour "power without limits" on one side, and "those who believe everything must be done in accordance with the law" - the reformists - on the other.
"The reformist movement always respects the framework of the law and non-violence," he asserted.
He also hit out at the Islamic republic's powerful religious leaders for turning the burgeoning youth population away from their faith and country.
"A great part of the youth are fleeing religion, in particular the social role of religion," the younger brother of the president said.
"I say clearly that when people are fleeing religion and the Islamic republic, the reason is a violent and dictatorial interpretation of religion."
The next parliamentary elections are scheduled for 20 February, 2004.
"Reformists are trying to prevent the present social movement from being transformed into a violent political revolt or into a scenario of an overthrow (of the government) from outside the country"
Muhammad Reza Khatami,
Iranian reformist leader
Led by the IIPF and rallying around President Muhammad Khatami, the reformists have controlled Iran's parliament since 2000, when they swept to power on a youth vote and a platform of shaking up the way Iran is governed.
But little of their agenda has made it into law, leading to major frustration among young people, students and women who rallied behind the movement.
The cause, reformists complain, is the overwhelming power wielded by conservatives in the judiciary, state media, security forces and legislative watchdogs.
Initiatives passed by a parliament are stymied by conservatives who see them as trying to undermine the foundations of the state.
An attempt by parliament to give greater powers to the president and strip conservative oversight bodies of their right to vet electoral candidates, also appears to have failed.
Amid widepsread frustration with the deadlock, voters showed their disdain in February 2003, when municipal elections saw an all-time low turnout for a country where voter participation regularly exceeds three-quarters of the electorate.
With just a tiny percentage of people bothering to cast their ballots, conservatives - relying on a committed hardcore support base - won the day.
Analysts see the very same happening in February. Some radical reformers have even called for a boycott of the elections, taking a stand against conservatives and forcing a political crisis rather than lose the elections.
But a number of IIPF delegates said the issue of a boycott would probably not yet be decided during the congress, as they were not yet certain that frustration among their supporters would lead to a low turnout.
One delegate said a low turnout may be expected in cities such as Tehran, but that reformists were still optimistic of strong support in provincial areas.