Iranian security forces have reportedly fired live ammunition and tear gas at demonstrators protesting against the Islamic Republic's initial denial it shot down a Ukrainian jetliner.

Key points: An online video showed crowds of demonstrators fleeing shots fired by police

An online video showed crowds of demonstrators fleeing shots fired by police Tehran's police chief General Hossein Rahimi denied his officers opened fire

Tehran's police chief General Hossein Rahimi denied his officers opened fire Crowds had been protesting against Iran's initial denial it shot down a Ukrainian jetliner

Videos sent to the New York-based Centre for Human Rights in Iran and later verified by The Associated Press showed a crowd of demonstrators fleeing as a tear gas canister landed among them.

The chief of police in the Iranian capital of Tehran, Hossein Rahimi, said officers are under orders to show restraint towards protesters.

He has denied police used live ammunition against demonstrators earlier this week, despite videos of the protests posted on social media in which gunshots can be heard.

People coughed while struggling to escape the fumes, with one woman calling out in Farsi: "They fired tear gas at people! Azadi Square. Death to the dictator."

Another video shot in the aftermath shows a woman being carried away, leaving a bloody trail.

People are seen around her crying out that she was shot in the leg with live ammunition.

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"Oh my God, she's bleeding nonstop," one person shouts. Another shouts: "Bandage it."

Photos and video after the incident showed pools of blood on the footpath.

However, Tehran's police chief General Hossein Rahimi later denied his officers opened fire.

"Police treated people who had gathered with patience and tolerance," General Rahimi told Iranian media.

"Police did not shoot in the gatherings since broad-mindedness and restraint has been the agenda of the police forces of the capital."

However, uniformed police officers were just one arm of Iran's security forces who were out in force for the demonstrations.

Riot police in black uniforms and helmets gathered earlier on Sunday in Vali-e Asr Square, at Tehran University and at other landmarks.

Revolutionary Guard members patrolled the city on motorbikes, and plainclothes security men were also out in force.

In November the guard was accused of opening fire on people protesting against government-set petrol price rises in violence that reportedly left more than 300 people dead.

Meanwhile, Stephane Dujarric, spokesman for United Nations secretary-general Antonio Guterres, said the reports of violence against those protesting the downing of a civilian airliner were "worrying".

"We're obviously following very closely the demonstrations that have been taking place today and over the weekend in Iran and the secretary-general recalls the rights to freedom of expression and association in peaceful assembly of people," Mr Dujarric said.

Protesters mourn the dead

The crash of the Ukraine International Airlines flight early on Wednesday killed all 176 people on board, mostly Iranians and Iranian-Canadians.

After pointing to a technical failure and insisting for three days that the Iranian armed forces were not to blame, authorities on Saturday admitted accidentally shooting it down in the face of mounting evidence and accusations by Western leaders.

Iran downed the flight as it braced for possible American retaliation after firing ballistic missiles at two bases in Iraq housing US forces earlier on Wednesday.

Iranian demonstrators defied a heavy police presence to protest their country's days of denials that it shot down a Ukrainian passenger plane. ( AP: Center for Human Rights in Iran )

The missile attack, which caused no casualties, was a response to the killing of General Qassem Soleimani, Iran's top general, in a US airstrike in Baghdad.

Iranians have expressed anger over the downing of the plane and the misleading explanations from senior officials in the wake of the tragedy.

They are also mourning the dead, which included many young people who were studying abroad.

At earlier protests on Saturday, students in Tehran shouted: "They are lying that our enemy is America! Our enemy is right here!"

Javad Kashi, a professor of politics at Tehran Allameh University, wrote online that people should be allowed to express their anger in public protests.

"Buckled under the pressure of humiliation and being ignored, people poured into the streets with so much anger," he wrote.

"Let them cry as much as they want."

AP/ABC