Iran's clerical establishment is under mounting pressure from a new bout of unrest over economic privations caused by inflation that has rocketed since last June.
Despite repeated outbreaks of nationwide protests stretching back decades, Iran's opposition has remained fragmented among rival groups and ideological factions and appears to have little organized presence inside the Islamic Republic.
Here are some opposition groups or blocs:
Monarchists
Iran's last shah, Mohammed Reza Pahlavi, fled in 1979 as the Islamic Revolution took hold. He died in Egypt in 1980.
His son, Reza Pahlavi, was heir to the Peacock Throne when the dynasty was ousted and, now based in the US, has called for regime change through non-violent civil disobedience, continued protests, and a referendum on a new government.
However, while Pahlavi has plenty of admirers in the Iranian diaspora who support a return to the monarchy, it is uncertain how popular that idea might be inside the country.
Meanwhile, there are splits even among pro-monarchist groups.
People's Mujahideen Organization
The Mujahideen were a powerful leftist group that staged bombing campaigns against the Shah's government and US targets in the 1970s, but ultimately fell out with the other factions.
The group is often known by its Persian name, the Mujahideen-e Khalq Organisation, or by the acronyms MEK or MKO.
Many Iranians, including sworn enemies of the Islamic Republic, cannot forgive it for siding with Iraq against Iran during the war of 1980-88.
The group was the first to publicly reveal in 2002 that Iran had a secret uranium-enrichment program, but it has shown little sign of any active presence inside Iran for years.
In exile, its leader, Massoud Rajavi, has not been seen for more than 20 years, and his wife, Maryam Rajavi, has taken control. Rights groups have criticized it for what they call cult-like behavior and abuses of its followers, which the group denies.
The group is the main force behind the National Council of Resistance of Iran, led by Maryam Rajavi, which has an active presence in many Western countries.
Ethnic minority groups
Iran's mostly Sunni Muslim Kurdish and Baluch minorities have often chafed against rule from the Persian-speaking, Shi'ite Muslim government in Tehran.
Several Kurdish groups have long organized opposition to the Islamic Republic in the western parts of the country where they form a majority, and there have been periods of active insurgency against government forces.
In Baluchistan, along Iran's eastern border with Pakistan, opposition to Tehran ranges from supporters of Sunni clerics seeking to carve out more space for their followers within the Islamic Republic to armed jihadists linked to al Qaeda.
When major rounds of protest have spread across Iran, they have often been fiercest in Kurdish and Baluchi areas, but in neither region is there a single, unified opposition movement that poses a clear threat to Tehran's rule.
Protest movements
Hundreds of thousands of Iranians have taken to the streets in mass protests at successive points for decades.
After the 2009 presidential election, demonstrators filled Tehran and other cities, accusing the authorities of rigging the vote for the incumbent Mahmoud Ahmadinejad against rival candidate Mir Hossein Mousavi.
Mousavi's "Green Movement" was crushed, and he was put under house arrest, along with political ally and former parliament speaker Mehdi Karoubi.
The movement, which sought democratic reform within the existing system of the Islamic Republic, is now widely seen as defunct.
In 2022, major protests again gripped Iran, centered on women's rights. The Woman, Life, Freedom demonstrations continued for months, but without resulting in an organization or leadership, and many of the protesters were ultimately arrested and jailed.