The pro-government marches followed the strongest warning yet from authorities when the army said it would confront "the enemies" behind the unrest - a move that could signal the kind of crackdown that has crushed protests in the past.
The crowds condemned the anti-government protesters as "Israel's soldiers", live state television coverage showed.
"Offenders of the Koran must be executed," they chanted.
The Twitter account 1500tasvir, which has 117,000 followers, reported heavy clashes in the central city of Isfahan between anti-government protesters and security forces.
It also showed anti-government street protests in several parts of the capital and in Shahin Shahr in central Iran.
State TV said 35 people had been killed in the unrest so far based on its own count and an official figure would be announced.
Many Iranians are fuming over the case of Mahsa Amini, 22, who died last week after being arrested by the morality police for wearing "unsuitable attire".
The morality police, attached to Iran's law enforcement, are tasked with ensuring the respect of Islamic morals as described by the country's clerical authorities.
Amini's death has reignited anger over issues including restrictions on personal freedoms in Iran, strict dress codes for women and an economy reeling from sanctions.
The anti-government protests are not expected to pose an immediate threat to Iran's clerical rulers, who have security forces which have put down one protest after another in recent years, analysts say.
But the protests have clearly made the authorities nervous. Women, who have played a prominent role, have challenged the country's Islamic dress code, waving and burning their veils.
Some have publicly cut their hair as furious crowds called for the fall of Supreme Leader Ayatollah Ali Khamenei.
Iran's police chief Hossein Ashtari weighed in with tough words in an attempt to stop the protests.
"The people's security is our red line," he told state TV. "Those involved in sabotage and creating insecurity based on directives from outside the country should know that they will be strongly dealt with."
The army's message on Friday, seen as a warning to protesters, read: "These desperate actions are part of the evil strategy of the enemy to weaken the Islamic regime."
The military said it would "confront the enemies' various plots in order to ensure security and peace for the people who are being unjustly assaulted".
Intelligence Minister Mahmoud Alavi on Friday also warned "seditionists" that their "dream of defeating religious values and the great achievements of the revolution will never be realized", according to the AsrIran website.
Friday's pro-government demonstrations showed the strength of the Islamic Republic, President Ebrahim Raisi said, adding that turmoil would not be tolerated.
"The people's presence (in the marches) today, is the power and the honour of the Islamic Republic," Raisi, facing the biggest protests since 2019, said on live television after returning from New York where he attended the United Nations General Assembly.
U.N. Secretary-General Antonio Guterres met with Raisi in New York on Thursday and raised human rights issues, a U.N. spokesperson said.
The United Nations is concerned "about reports of peaceful protests being met with excessive use of force leading to dozens of deaths and injuries", spokesperson Stephane Dujarric told reporters.
Human rights group Hengaw said a general strike was held on Friday in Oshnavieh, Javanroud, Sardasht and other towns in the northwest where many of Iran's up to 10 million Kurds live.
Internet blockage watchdog NetBlocks said mobile internet had been disrupted in Iran for a third time.
Twitter accounts linked to Anonymous "hacktivists" voiced support for the protests and said they had attacked 100 Iranian websites, including several belonging to the government.
Websites of the central bank, Supreme Leader Ayatollah Ali Khamenei and several state-affiliated news agencies have been disrupted in recent days.
Iran's clerical rulers fear a revival of the protests that erupted in 2019 over gasoline price rises, the bloodiest in the Islamic Republic's history. Reuters reported 1,500 people were killed.
Rights groups such as Hengaw and HRANA, lawyers and social media users reported widespread arrests of students and activists at their homes by security forces in an apparent effort to curb protests.
Majid Tavakoli, a student leader turned human rights activist, was detained overnight, his brother Mohsen said.
"They raided the home and arrested Majid while he was asleep ... We are unable to do anything. Please spread the word," Mohsen Tavakoli tweeted.