Iranian demonstrators defied a heavy police presence Sunday night to protest their country's days of denials that it shot down a Ukrainian passenger plane carrying 176 people, the latest unrest to roil the capital amid soaring tensions with the United States.

Videos posted online showed protesters shouting anti-government slogans and moving through subway stations and sidewalks, many near Azadi Square after an earlier call for people to demonstrate there.

Other videos suggested similar protests were taking place in other Iranian cities.

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

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

People looked down as they walked briskly past police, hoping not to draw attention to themselves.

The plane crash early Wednesday killed everyone on board, mostly Iranians and Iranian-Canadians.

After initially pointing to a technical failure and insisting the armed forces were not to blame, authorities on Saturday admitted to 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.

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. But no retaliation came.

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.

Even talking about it makes my heart beat faster and makes me sad, said Zahra Razeghi, a Tehran resident. I feel ashamed when I think about their families.

“The denial and covering up the truth over the past three days greatly added to the suffering and pain of the families, and me, she added.

Another individual, who identified himself only as Saeed, said Iran's largely state-run media had concealed the cause of the crash for political reasons.

Later developments changed the game, and they had to tell the truth,” he said.

Earlier Sunday, hundreds of students gathered at Tehran's Shahid Beheshti University to mourn the victims and protest against authorities for concealing the cause of the crash, the semiofficial ISNA news agency reported.

Bahareh Arvin, a reformist member of the Tehran City Council, took to social media to say she was resigning in protest at the government's lies and corruption.

“With the current mechanism, there is no hope of reform,” she said.

Some Iranian artists, including famed director Masoud Kimiai, withdrew from an upcoming international film festival.

Two state TV hosts resigned in protest over the false reporting about the cause of the plane crash.

President Donald Trump, who has expressed support for past waves of anti-government demonstrations in Iran, addressed the country's leaders in a tweet, saying DO NOT KILL YOUR PROTESTERS.

The World is watching. More importantly, the USA is watching, he tweeted.

Iranians took to the streets in November after the government hiked gas prices, holding large protests in several cities.

The government shut down internet access for days, making it difficult to gauge the scale of the protests and the subsequent crackdown.

Amnesty International later said more than 300 people were killed.

A candlelight ceremony late Saturday in Tehran turned into a protest, with hundreds of people chanting against the country's leaders including Supreme Leader Ayatollah Ali Khamenei and police dispersing them with tear gas.

Protests were also held in the city of Isfahan and elsewhere.

Police briefly detained the British ambassador to Iran, Rob Macaire, who said he went to the vigil without knowing it would turn into a protest.

Can confirm I wasn't taking part in any demonstrations! he tweeted. Went to an event advertised as a vigil for victims of #PS752 tragedy.

Normal to want to pay respects some of victims were British. I left after 5 mins, when some started chanting. He said he was arrested 30 minutes after leaving the area.

Britain said its envoy was detained without grounds or explanation and in flagrant violation of international law. Iran's Deputy Foreign Minister Abbas Araghchi later tweeted that Macaire was arrested as an unknown foreigner in an illegal gathering.”

Araghchi said when police informed him that a man was arrested who claimed to be the British ambassador, he didn't believe them.

But he said that once he spoke to Macaire by phone, he realized it was him, and that the ambassador was freed 15 minutes later.

Iran's Foreign Ministry later summoned the British ambassador over his illegal and inappropriate presence” at the protest, it said on its Telegram channel.

Alaeddin Boroujerdi, a member of Iran's parliamentary committee on national security and foreign policy, accused the ambassador of organizing protests and called for his expulsion.

Dozens of hard-liners later gathered outside the British Embassy, chanting Death to England.

They also called for the ambassador to be expelled and the embassy to be closed.

Iranian media, meanwhile, focused on the admission of responsibility for the crash, with several newspapers calling for those responsible to apologize and resign.

The hard-line daily Vatan-e Emrouz bore the front-page headline A sky full of sadness, while the Hamshahri daily went with Shame, and the IRAN daily said Unforgivable.

Mehdi Karroubi, an opposition activist under house arrest, lashed out at Khamenei himself, saying that as commander in chief he was directly responsible.

If you were aware and you let military and security authorities deceive people, then there is no doubt you lack the attributes of constitutional leadership,” he said in a statement.

Criticism of the supreme leader is punishable by up to two years in prison.

Tensions with the United States eased after the ballistic missile attack, when Trump declined to respond and welcomed Iran's apparent decision to stand down.

The emir of Qatar, Sheikh Tamim bin Hamad Al Thani, met with Iranian President Hassan Rouhani and other officials Sunday in Tehran. Qatar has warm relations with the US and Iran.

Syrian Prime Minister Imad Khamis was also leading a high-level delegation to Iran, including the defence and foreign ministers. Syrian state media described it as an important visit in light of recent events, without elaborating.

Iran is a key ally of Syrian President Bashar Assad in his country's civil war, and Soleimani had mobilized militias and coordinated military aid.

Pakistan's Foreign Minister Shah Mahmood Qureshi was also traveling to Iran, with plans to visit Saudi Arabia the following day.

And Japanese Prime Minister Shinzo Abe was in Saudi Arabia for talks with King Salman as part of a tour of oil-producing Gulf Arab states aimed at promoting peace.