Students at Tehran University today protested the death sentence of a popular professor for a fourth straight day in a mounting challenge against the conservative judiciary that has sought to prevent liberalizing reforms in Iran.

The protests were described by sources in Tehran as the most intense public demonstrations since 1999, when students protesting the closure of a newspaper were harshly put down by informal militias tied to the government. Observers in the Iranian capital reached by telephone said the new round of protests appeared to be spontaneous, beginning as a complaint against the quality of cafeteria food before taking up the cause of the condemned professor.

Several thousand young people have participated in Tehran, the sources said, and the protests were reported to have spread to the provincial cities of Hamadan, Oroumieh, Isfahan and Tabriz. Police so far have not cracked down.

Hashem Aghajari, a history professor and prominent reformist politician allied with President Mohammad Khatami, was sentenced to death by hanging last week after being convicted of insulting the prophet Muhammad during a speech in which he questioned the authority of conservative clerics who wield decisive power in Iran. The decision was picked up by the students as a symbol of the judiciary's power to impose its conservative views on this nation of 66 million.

"Execution of Aghajari is execution of thought in Iran!" the students chanted in protests that witnesses and news reports said extended past midnight. "Our problem is the judiciary!"

Two decades after Ayatollah Ruhollah Khomeini led a mass uprising against the secular, U.S.-backed rule of Shah Mohammad Reza Pahlavi, the Islamic Republic of Iran has become in many respects a divided country. Hard-line conservative clerics hold powerful appointive positions in the government, including in the judiciary and military, and insist on maintaining the most stringent rules of Islamic behavior. Atop the theocracy stands Ayatollah Ali Khamenei, the supreme leader chosen by Khomeini as his successor.

A number of reformers, many of whom are also clerics, control the presidency and parliament and are leagued against the hard-liners. Swept into office by electoral landslides in 1997 and 2001, the reformers favor loosening strictures on dress and behavior and reducing Iran's international isolation in hopes of stimulating its economy. Their core constituency is people who were born after the Islamic Revolution in 1979, now a majority of the population, and would like to see its leadership evolve.

"The students traditionally do represent the thinking of the majority," said Shirzad Bozorgnehr, editor of the Iran News. "They are holding what they call a street tribunal. A podium is set up and people come and talk, mostly students. So far it's been peaceful, [but] it's like a tinderbox."

But despite the support they have earned in the elections, reformers in parliament and the executive branch have been stymied by the judiciary. Conservative clerics routinely jail journalists, politicians and other clerics deemed threats to the system.

After five years of frustration, Khatami last month introduced legislation designed to bring the jurists under his control. One bill moving through parliament would allow the president to suspend any judicial ruling he deemed unconstitutional. A second would remove clerics' authority to decide who can stand for election. Both measures are expected to pass easily. But before becoming law, the bills must be ruled on by bodies dominated by the very people whose power they would reduce: those who sit on the Guardian Council, followed by members of the Expediency Council.

Analysts said Khatami thus has little choice but political confrontation after trying in vain to nudge Iran's theocracy toward liberalization by more discreet methods. The president's private weekly meetings with the supreme leader have produced little, reformers say.

"The fact that Khatami has been pushing these two bills shows behind-the-scenes dialogue hasn't worked, and they need to be more upfront about pushing their agenda," said a Western diplomat, speaking by telephone from Tehran. "The regime is showing signs of strain and the stakes are very high right now."

The State Department spokesman, Richard Boucher, said Aghajari's sentence represents "a breach of accepted international standards of due process."

He added: "We're gravely concerned about the case, which indicates a deteriorating human rights situation in Iran. Public executions, stonings, punitive amputations and the persecution of reformers and the press have increased over the last several months. The United States stands with the people of Iran in their quest for greater freedom, prosperity, judicial due process and the rule of law."

The constitutional showdown became the backdrop for the student protests. With the judiciary already in the spotlight, outrage over the harsh sentence for Aghajari, by a provisional judge in Hamadan, appeared to damage the conservatives' standing, analysts said. Two members of parliament from Hamadan announced they were resigning in protest. The speaker of the parliament, Mehdi Karrubi, expressed disgust at the verdict in a live radio address. In a chant heard Monday, students urged Khatami to resign if he proves unable to push forward his agenda.

"There's a strong reaction from even inside the government, which is new," said a European diplomat in Tehran, who suggested the hard-liners are losing ground. "All these things are linked together."

Iranian students protesting in Tehran hold pictures of Hashem Aghajari, a professor who was sentenced to death for insulting the prophet Muhammad.A university student yells during a fourth day of demonstrations against the country's judiciary.