Iran's leaders are facing growing calls to dismiss senior officials after a Ukrainian passenger plane was shot down killing all 176 people on board.
Thousands of protesters demanded accountability on Saturday after the military said it had mistakenly downed the jet, having earlier denied it.
Riot police have been deployed and there are reports that protesters have gathered for a second day of action.
The plane was shot down amid rising tensions with the US.
It happened shortly after Iran launched missiles at two airbases housing US forces in Iraq. Those strikes were a response to the US killing of senior Iranian commander Qasem Soleimani in a drone strike in Baghdad on 3 January.
Dozens of Iranians and Canadians, as well as nationals from Ukraine, the UK, Afghanistan and Sweden died on the plane.
It was en route to the Ukrainian capital, Kyiv, but came down near Imam Khomeini Airport in Tehran shortly after take-off.
What's the latest?
Riot police have been mobilised on the streets of Tehran in an effort to deter more demonstrations.
Despite this, videos circulating online appear to show demonstrators gathering for renewed protests on Sunday morning. In some of the clips, protesters can be heard chanting anti-government slogans.
Demonstrators have gathered in other cities as well as in the capital, reports say.
A number of Iranian newspapers have covered the vigils for the victims of the disaster alongside headlines such as "Shame" and "Unforgivable".
But there has also been praise for what one pro-government newspaper called Iran's "honest" admission of error.
Those who decide to continue demonstrating will be mindful of the violence with which the security forces have dealt with protest movements in the past. the BBC's Arab affairs editor Sebastian Usher says.
On Saturday, students gathered outside two universities. They initially did so to pay respect to the victims, but angry protests erupted later in the evening.
The students called for those responsible for the downing the plane, and those they said had covered up the action, to be prosecuted.
Social media users also vented anger at the government's actions.
What has the international reaction been?
US President Donald Trump on Sunday repeated warnings that Iran should not target anti-government protesters, saying, "the World is watching. More importantly, the USA is watching".
To the leaders of Iran – DO NOT KILL YOUR PROTESTERS. Thousands have already been killed or imprisoned by you, and the World is watching. More importantly, the USA is watching. Turn your internet back on and let reporters roam free! Stop the killing of your great Iranian people!
— Donald J. Trump (@realDonaldTrump) January 12, 2020
End of Twitter post by @realDonaldTrump
Britain, meanwhile, has condemned the arrest of the UK ambassador to Iran in Tehran as a "flagrant violation of international law".
Foreign Secretary Dominic Raab said Rob Macaire was detained after attending a vigil where he was paying respects to victims of the crash, some of whom were British.
Mr Macaire said he left the vigil when some people started chanting and had played no part in the demonstration.
Iran on Sunday summoned the ambassador to complain about "his unconventional behaviour of attending an illegal rally", the foreign ministry website said.
Iranian protesters set a UK flag alight in front of the UK embassy on Sunday.
Increasing list of grievances
Analysis by Dr Aniseh Bassiri Tabrizi, Royal United Service Institute
The Iranian people will probably demand accountability and the prosecution of those responsible for downing the plane, as well as the adoption of all the steps needed to ensure this does not happen again.
They will also pay attention to how the victims of the air crash are treated by the Iranian elite. An important test here is whether their funerals will result in national mourning, similar to that of Soleimani, or instead be largely ignored.
All of these demands will be added to previous grievances over the state of the economy and the limitations on some social freedoms.
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