A group of Iranian protesters has demanded Iran's Supreme Leader Ayatollah Ali Khamenei and other senior leaders step down after Tehran admitted its forces mistakenly shot down a Ukrainian passenger plane, killing all 176 people on board.

"Commander-in-chief [Khamenei] resign, resign," videos posted on Twitter showed hundreds of people chanting in front of Tehran's Amir Kabir University on Saturday.

Protesters chanted slogans denouncing "liars" and demanded the resignation and prosecution of those responsible for downing the plane and allegedly covering up the accidental action.

Others on Twitter asked why the Boeing 737-800 was allowed to take off on Wednesday, at a time when tensions between the United States and Iran were so high - Ukraine International Airlines Flight 752 crashed moments after departing from Tehran, hours after Iran struck bases housing US troops in neighbouring Iraq in retaliation to the US assassination of a top Iranian commander last week.

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Fars news agency reported that Iranian police dispersed students that were chanting "destructive" and "radical" slogans during the gathering in the capital.

The United Kingdom confirmed its ambassador, Rob Macaire, was arrested briefly during the demonstrations, accused of "inciting" the protesters in front of the Amir Kabir University. Foreign Secretary Dominic Raab said the arrest was a "flagrant violation of international law" and repeated calls for Iran to de-escalate tensions.

Earlier on Saturday, after days of denials, Iran said its military had shot down the Ukrainian plane, calling it a "disastrous mistake" but blaming the US for heightened tension.

The military claimed air defences were fired in error during an alert which was imposed after the missile struck against US targets in Iraq. Authorities promised to bring those responsible to justice.

Speaking from Tehran, Al Jazeera's Dorsa Jabbari said: "There is a lot of anger. Iranians are demanding justice and accountability. Many people including families of the victims are in shock. They do not understand why their government would have lied to them for this long."

"Vigils that were held near Amir Kabir University quickly turned into anti-government protests with people calling for the IRGC to leave the country," she said, referring to the elite Islamic Revolution Guard Corps.

Iran's leadership last faced mass protests in November following the rise in petrol prices. The country has been in the grip of a severe economic crisis since President Donald Trump withdrew in 2018 the US from the nuclear deal signed between Iran and world powers in 2015 and reimposed punishing sanctions.

Condemnations

The plane, bound for the Ukrainian capital of Kiev, was carrying 167 passengers and nine crew members from several countries, including 82 Iranians, 57 Canadians - including many Iranians with dual citizenship - and 11 Ukrainians, according to officials.

Foreign governments condemned the downing of the plane, with Ukraine demanding compensation and a United States official calling the action "reckless".

Canadian Prime Minister Justin Trudeau called for "a complete and thorough investigation" with Iran's full cooperation, while the UK said Tehran's admission was an important first step.

Supreme leader Khamenei, until now silent about the crash, said information should be made public, while top officials and the military issued apologies.

But state television suggested revealing the truth might be used by the "enemies of Iran", usually a reference to the US and Israel.

Experts said mounting international scrutiny would have made it all but impossible to hide signs of a missile strike in any investigation.

They said Iran may have felt a U-turn was better than battling rising criticism abroad and growing grief and anger at home, as many victims were Iranians with dual nationality.

"There's nothing you can do to cover it up or hide it," said Anthony Brickhouse, an air safety expert at Embry-Riddle Aeronautical University and former US National Transportation Safety Board investigator. "Evidence is evidence."

The crash heightened international pressure on Iran after months of friction with the US and tit-for-tat attacks.

A US drone strike killed top Iranian military commander Qassem Soleimani in Iraq on January 3, prompting Tehran to fire at US targets on Wednesday.

Following the killing of Soleimani, a hero for many in Iran, huge crowds across the country attended funeral procession for the slain head of the IRGC's overseas forces in an apparent show of support for the country's leadership.

Rare apology

In a rare step, IRGC on Saturday apologised to the nation and accepted full responsibility for the plane crash.

Senior IRGC commander Amir Ali Hajizadeh said he had informed Iran's authorities on Wednesday about the unintentional strike, a comment that raised questions about why officials had publicly denied it for so long.

Speaking on state television, he said he wished he "could die" when he heard the news about the incident.

Iranian Foreign Affairs Minister Mohammad Javad Zarif wrote on Twitter that "human error at time of crisis caused by US adventurism led to disaster", citing an initial armed forces investigation into the crash.

A military statement said the plane flew close to a sensitive IRGC site at a time of high alert.

But Ukraine said the plane was in a normal flight corridor. Iran's Civil Aviation Organisation also said the airliner had not veered off its normal course.

Ukraine International Airlines said Iran should have closed the airport, adding that it received no indication it faced a threat and was cleared for take-off.

The disaster echoed a 1988 incident, when a US warship shot down an Iranian airliner, killing 290 people. While Washington claimed it was an accident, Tehran said it was intentional.