TEHRAN, Iran - Thousands of students ignored official warnings and demonstrated here yesterday for a fourth day over the death sentence for a reformist scholar charged with apostasy.

About 5,000 students gathered at Tehran University in support of the academic Hashem Aghajari, sentenced to hang for questioning clerical rule in the Islamic Republic.

"The execution of Aghajari is the execution of the university!" demonstrators chanted. "Political prisoners should be freed!"

The momentum of protests appeared to be growing, with more students gathering in Tehran each day and demonstrations spreading to the provincial cities of Tabriz, Isfahan, Urumiyeh and Hamedan.

After their rally in Tehran, students marched through the huge university campus, holding hands and singing "Ey Iran," the national anthem before the 1979 Islamic revolution.

The watching police did not intervene.

Iran's supreme leader and commander of the armed forces, Ayatollah Ali Khamenei, issued a veiled warning late Monday about the possible use of force if Parliament, the government and judiciary cannot settle their differences.

Using a term that has been widely used to describe the Revolutionary Guards and other hard-line militia groups, he said: "The day the three branches are unable or unwilling to settle major problems, the leadership will, if it thinks it necessary, use the popular forces to intervene."

"I hope that will never happen," he added, in comments broadcast on television.

Today's protests followed similar demonstrations Monday at Modaress Training University in the central part of the city. More than 1,000 students carried a portrait of Aghajari as they marched and chanted demands for the resignation of Mahmoud Hashemi Shahroudi, the hard-line chief of the Iranian judiciary. The students also called for the resignation of President Mohammad Khatami, the leader of Iran's reform movement, in a sign of frustration with the continuous setbacks for reform.

Aghajari was sentenced to death last week in a closed-door trial in Hamedan on charges that he had insulted the Prophet Muhammad. The charges stemmed from a speech he made in August in which he called on people to not follow religious leaders blindly.

The demonstrations have been the largest since 1999, when students staged a week of protest throughout the country after hard-line vigilantes attacked a student dormitory.

A leading student activist, Saeed Razavi-Faqih, told the Iranian Student News Agency on Monday that students had decided to continue their protests in order to confront constitutional violations, not only in Aghajari's case but also in general.

"We must reach a stage in our destiny that we have lawful rights and freedoms," Razavi-Faqih was quoted as saying.