The US says "the world is watching" how Iranian authorities respond to anti-government protests that have broken out in several cities.
Police dispersed anti-government demonstrators in the western city of Kermanshah as protests spread to Tehran and several other cities a day after rallies in the northeast, the semi-official news agency Fars said.
The outbreak of unrest reflects growing discontent over rising prices and alleged corruption, as well as concern about the Islamic Republic's costly involvement in regional conflicts such as those in Syria and Iraq.
The protests were initially against economic conditions and corruption but appear to have turned political. They started in the north-eastern city of Mashhad - the country's second most-populous - on Thursday.
People there took to the streets to express anger at the government over high prices, and vented their fury against President Hassan Rouhani. Fifty-two people were arrested for chanting "harsh slogans".
Demonstrations of varying sizes are reported to have occurred in at least seven cities, with thousands turning out in Kermanshah, Rasht, Isfahan and Qom on Friday. In Tehran, social media footage showed a heavy police presence.
About 300 demonstrators gathered in Kermanshah after what Fars said was a "call by the anti-revolution". They shouted: "Political prisoners should be freed" and "Freedom or death", and some public property was destroyed. Fars did not name any opposition groups.
An official said a few protesters had been arrested in Tehran, and footage posted on social media showed a heavy police presence in the capital and some other cities.
Slogans have been chanted against not just Mr Rouhani but Supreme Leader Ayatollah Ali Khamenei, and clerical rule in general.
Rouhani's leading achievement, a 2015 deal with world powers that curbed Iran's disputed nuclear program in return for a lifting of most international sanctions, has yet to bring the broad economic benefits the government says are coming.
Unemployment stood at 12.4 percent this fiscal year, according to the Statistical Centre of Iran, up 1.4 percentage points from the previous year. Out of a population of 80 million, about 3.2 million Iranians are jobless.
Demonstrators were reportedly heard yelling slogans like "The people are begging, the clerics act like God". Protests have even been held in Qom, a holy city home to powerful clerics.
There is also anger at Iran's interventions abroad. In Mashhad, some chanted "not Gaza, not Lebanon, my life for Iran", a reference to what protesters say is the administration's focus on foreign rather than domestic issues.
Other demonstrators chanted "leave Syria, think about us" in videos posted online. Iran is a key provider of military support to the government of Bashar al-Assad in Syria.
It is also accused of providing arms to Houthi rebels fighting a Saudi-led coalition in Yemen, which it denies, and is an ally of Lebanon's powerful Shia movement Hezbollah.
Openly political protests are rare in Iran, where security services are omnipresent.
On Saturday, annual nationwide pro-government rallies were held, marking the end of the unrest that shook the country in 2009, state media reported.
State television showed a rally in the capital Tehran and marchers carrying banners in support of Supreme Leader Ayatollah Ali Khamenei in Iran's second largest city of Mashhad just days after the protests.
US condemns protester arrests
Washington criticized the arrests.
"The United States strongly condemns the arrest of peaceful protesters. We urge all nations to publicly support the Iranian people and their demands for basic rights and an end to corruption," the US State Department said in a statement.
A White House statement said Iranians were fed up with "the regime's corruption and its squandering of the nation's wealth to fund terrorism".
"The Iranian government should respect their people's rights, including their right to express themselves. The world is watching," White House spokeswoman Sarah Huckabee Sanders said on Twitter.
The tweet later appeared on US President Donald Trump's Twitter account.
The BBC's Persian Service said thousands of people were likely to be bussed into a rally in the capital, Tehran.
Iran's First Vice-President Eshaq Jahangiri suggested government opponents were behind the protests, according to comments reported by state broadcaster IRIB.
"Some incidents in the country these days are on the pretext of economic problems, but it seems there is something else behind them. They think by doing this they harm the government, but it will be others who ride the wave," he said.
Earlier, Fars news agency reported protesters in Kermanshah had destroyed some public property and had been dispersed.
Tehran's governor-general said any such gatherings would be firmly dealt with by the police, who are out in force on the main roads.
Officials in Mashhad said the protest was organised by "counter-revolutionary elements", and video online showed police using water cannons.
There have been calls on social media for protests up and down the country, despite warnings from the government against illegal gatherings.
- Reuters / BBC