‘Hang them! Hang them!” the mob shouted while goose-stepping towards the podium. Some beat their chests and others raised clenched fists. “Allah is the greatest!” chanted the turbaned clerics.

This was the scene Tuesday in the Islamic Majlis, Iran’s ersatz parliament. Members raged against the “heads of sedition,” calling for their execution. By this they meant three of the most prominent leaders of Iran’s reformist Green Movement—former President Mohammad Khatami, former Prime Minister Mir Hossein Mousavi, and Mehdi Karroubi, a cleric and 2009 presidential candidate who chaired the Majilis just six years ago.

Sedition, or fitnah in Persian, is a Shariah term that describes action against the ruling Islamic authority. The regime uses it to justify its brutal crackdown on the pro-democracy movement triggered by the disputed re-election of President Mahmoud Ahmadinejad in June 2009.

The “sedition” trio had called for a march in Tehran on Monday, ostensibly to mark “the end of despotism in Egypt.” To do things legally, they applied for a permit, which the authorities refused. Meanwhile, the police blocked access to their homes and cut their telephone and Internet links. Not to leave things to chance, the authorities deployed more than 30,000 Basij (non-uniformed thugs) to help riot police contain any demonstrations.

Crowds formed in various parts of the capital with the aim of converging on Azadi (freedom) Square to turn it into an Iranian version of Cairo’s Tahrir (liberty) Square. The Basij blocked access, provoking intermittent battles with crowds of demonstrators throughout the day. Two students, Sanah Jaleh, 26, and Mohamad Mokhtari, 22, were killed. And at least 400 people, including nine Basij, were injured. Regime propaganda has blamed “the CIA and Mossad” for the uprising and claims that “Zionist snipers” shot the two students.

The exercise was a partial success for the opposition. The crowds in Tehran did not exceed a few thousand before they were dispersed, but the protesters appeared as determined as ever despite 18 months of ferocious repression. More importantly, perhaps, protest marches also took place in a dozen cities across the country. Nevertheless, in its present form the opposition is unlikely to topple the Khomeinist regime.

The Iranian opposition consists of a wide variety of parties and groups with different ideologies and goals. Messrs. Khatami, Mousavi and Karroubi miss few opportunities to declare that they are not seeking regime change but change within the regime. They claim that Mr. Ahmadinejad has abandoned “the true teachings” of the Ayatollah Ruhollah Khomeini, who founded the Islamic Republic in 1979.

The trio are often labelled “reformists” although they have not indicated a single reform they plan to introduce. Their main threat is that, unless they are allowed to form political parties, they will boycott the coming Majlis elections this summer.

A second group within the opposition consists of Khomeinist former officials who promote the so-called “Turkish model,” which would allow Iran to normalize ties with the West while building an Islamic system with the military as ultimate arbiter.

This group, including people such as former Deputy Premier Behzad Nabavi and former Foreign Minister Ibrahim Yazdi, is promoting the image of a green electric lamp to symbolize the opposition Green Movement. The image of a yellow electric lamp is the symbol of Turkey’s governing Justice and Development Party (AKP).

A third group, however, appears to have concluded that only regime change can save Iran from national catastrophe. Here we find people like former Deputy Interior Minister Mostafa Tajzadeh and former Majlis member Mohsen Armin.

Significantly, some of the strongest voices for regime change come from the clergy. Grand Ayatollahs Hassan Sanei and Asadollah Bayat have declared the regime’s founding principle of wilayat al-faqih (clerical rule) to be “contrary to Islam.” They have called for “a new constitution in which the people exercise the power that Allah gives human beings to manage earthly affairs.” The cleric and former interior minister Abdallah Noori has also called for “a new departure with the people in the driving seat.”

Monday’s marches showed that the undercurrent of Iranian politics may have become more radicalized than Messrs. Khatami, Mousavi and Karroubi believe. The most popular slogans were “Down with the Dictator!” and “Freedom! Freedom! Freedom!” Speeches were not about “reviving the teachings of Khomeini,” as the trio preach, but about regime change and free elections.

Many Iranians believe that the Tunisian and Egyptian uprisings that toppled two Arab despots were inspired by Iran’s pro-democracy demonstrations of 2009. This week’s protests revealed three things: Iran’s opposition movement is wounded but alive; it is united in its rejection of Mr. Ahmadinejad; and, slowly but surely, it is discarding the option of change within the regime and seeking to change the regime.

Mr. Taheri is the author of “The Persian Night: Iran Under the Khomeinist Revolution” (Encounter, 2009).