Wednesday's shooting down of Ukraine International Airlines flight 752, killing all 176 people on board, has led to one of the greatest public challenges to Iran's clerical rulers since they came to power in the Islamic Revolution of 1979.

After days of denying blame, Iran acknowledged on Saturday it had shot the plane down during a state of high alert, hours after it had fired missiles at US targets in Iraq to retaliate for the killing of Iran's most powerful military commander.

Protesters, with students at the forefront, have held demonstrations against the establishment since Saturday, some met by a violent police crackdown.

Video from inside Iran has shown wounded people being carried, pools of blood on the streets and the sound of gunfire.

President Hassan Rouhani promised a thorough investigation into the "unforgiveable error" of shooting down the plane, giving a television address on Tuesday.

Iran's judiciary spokesman Gholamhossein Esmaili said some of those accused of having a role in the plane disaster had already been arrested. He did not identify the suspects or say how many had been held.

Most of those on board the flight were Iranians or dual nationals. Canada, Ukraine, Britain and other nations who had citizens on the plane have scheduled a meeting on Thursday in London to consider legal action against Tehran.

The disaster and subsequent unrest comes amid one of the biggest escalations between Tehran and Washington since the revolution four decades ago that made them into foes.

Tit-for-tat military strikes began with missiles launched at a US base that killed an American contractor in December, and reached their climax when Washington killed the architect of Iran's regional network of proxy militia, Qassem Soleimani, in a drone strike in Baghdad on January 3.

In recent days, demonstrators have chanted "Clerics get lost!" and other slogans against Iran's system of theocratic rule.

Riot police have beaten some demonstrators with batons, social media posts show. Gunfire has also been heard, although police have denied shooting at protesters.

"Iranian armed forces admitting their mistake is a good first step," Rouhani said in Tuesday's address. He said those responsible would be punished and the government would be accountable to Iranians and those nations who lost citizens.

Ukraine's foreign minister said "grieving nations", five of the countries whose citizens were killed when an airliner was shot down, would meet on Thursday.

"We will meet in person in London to discuss the ways, including legal, (for) how we are following this up," Vadym Prystaiko said on Monday.

Canadian Prime Minister Justin Trudeau, whose nation had at least 57 citizens on the flight, told Global News TV that victims would still be alive and at home with their families now if there had been no tensions in the region.

According to a transcript, Trudeau said Canada did not receive a heads up before the United States killed Soleimani.

"The US makes its determinations. We attempt to work as an international community on big issues. But sometimes countries take actions without informing their allies," he said.

Australian Associated Press