Police in the Iranian capital, Tehran, have denied using live ammunition against protesters outraged by the shooting down of a Ukrainian airliner.
Officers had been given orders to "show restraint", the chief of police said.
Protests erupted on Saturday, after Iran admitted firing missiles by mistake at the Ukraine International Airlines jet that crashed near Tehran.
All 176 people on board flight PS752, mostly Iranians and Canadians, were killed.
For the first three days after the crash, Iran denied that its armed forces had shot down the plane and suggested there had been a technical failure.
The admission of responsibility, which came after video emerged of a missile appearing to hit the plane, provoked widespread anger in Iran against the establishment.
Days earlier, Iranians had been united in grief over the killing of Gen Qasem Soleimani, their country's second most powerful man, in a US drone strike in Iraq.
The latest reports of a possible crackdown echo the protests in Iran in November over rising fuel prices. Human rights groups say hundreds of people were killed.
Sunday's demonstrations went on late into the night, as people vented their fury against the Iranian government and the powerful Revolutionary Guards, who shot down the Ukrainian plane.
There were reports that a number of people were injured when security forces broke up a protest in Tehran's Azadi Square, during which people chanted "death to the dictator" - a reference to Supreme Leader Ayatollah Ali Khamenei.
One video posted on social media allegedly showed members of the paramilitary Basij Resistance Force, which is often used to suppress domestic dissent, attacking protesters in the area. What appears to be gunfire can be heard.