Protests erupted across Iran for a second day on Sunday, piling pressure on the leadership after the military admitted it had mistakenly shot down a Ukrainian airliner at a time when Tehran feared U.S. air strikes.

"They are lying that our enemy is America, our enemy is right here," a group of protesters outside a university in Tehran chanted, according to video clips posted on Twitter.

Posts showed other demonstrators outside a second university and a group of protesters marching to Tehran's Azadi (Freedom) Square. The videos also showed protests in other cities.

Some state-affiliated media carried reports of the university protests, after Saturday's demonstrations sparked by Iran's announcement that its military had mistakenly brought down the Ukrainian plane on Wednesday, killing all 176 aboard.

Tehran residents told Reuters police were out in force in the capital on Sunday. Public anger boiled up following days of denials by the military that it was to blame, issued even as Canada and the United States said a missile had brought the plane down.

Riot police fired teargas on Saturday at thousands of protesters in the capital, where many chanted "Death to the dictator", directing their anger at the Islamic Republic's Supreme Leader Ayatollah Ali Khamenei.

"Apologise and resign," Iran's moderate Etemad daily wrote in a banner headline on Sunday, saying the "people's demand" was for those responsible for mishandling the plane crisis to quit.

The latest upsurge in anger adds to public pressure on the authorities, which is struggling to keep the crippled economy afloat under stringent U.S. sanctions.

It launched the bloodiest crackdown in the 40-year history of the Islamic Republic in November after protests against a hike in fuel prices turned political. About 1,500 people were killed in less than two weeks of unrest, three interior ministry officials told Reuters, though international rights groups put the figure much lower and Iran rejected the figure.

U.S. President Donald Trump tweeted on Sunday: "To the leaders of Iran - DO NOT KILL YOUR PROTESTERS. Thousands have already been killed or imprisoned by you, and the World is watching."