DUBAI - Iran's admission that it shot down a Ukrainian airliner, killing all 176 aboard, has provoked international outrage and triggered protests against Iranian authorities in Teheran and other cities including one in which Britain's ambassador was detained.

In what President Hassan Rouhani called a "disastrous mistake", Iran said on Saturday (Jan 11) that a missile fired in error on Wednesday by its air defences while on alert after Iranian missile strikes on US targets in Iraq brought down the plane.

Iran had denied for days after the crash that it had shot down the airliner.

Even as top Iranian officials and the military issued apologies, protests against authorities spread across Iran including in the capital Teheran, Shiraz, Esfahan, Hamedan and Orumiyeh.

US President Donald Trump, who has said he does not seek "regime change" in Iran, took to Twitter to express his support for the demonstrators, writing, "We are following your protests closely, and are inspired by your courage."

"The government of Iran must allow human rights groups to monitor and report facts from the ground on the ongoing protests by the Iranian people. There can not be another massacre of peaceful protesters, nor an internet shutdown. The world is watching," Trump wrote.

Britain's Foreign Office confirmed late on Saturday that the country's ambassador in Teheran had been briefly detained by Iranian authorities. The Teheran-based Tasnim news agency said the envoy was arrested for several hours in front of Amir Kabir University for inciting anti-government protesters.

"The arrest of our Ambassador in Teheran without grounds or explanation is a flagrant violation of international law,"Foreign Secretary Dominic Raab said in a statement.

"The Iranian government is at a cross-roads moment. It can continue its march towards pariah status with all the political and economic isolation that entails, or take steps to de-escalate tensions and engage in a diplomatic path forwards,"Raab added.

A leader of Iran's opposition Green Movement, Mehdi Karroubi, called on Iran's Supreme Leader Ayatollah Ali Khamenei to step down over the handling of the downed airliner.

Foreign governments condemned the downing of the plane, with Ukraine demanding compensation. Canada, Ukraine and Britain, however, called Teheran's admission an important first step.

"What Iran has admitted to is very serious. Shooting down a civilian aircraft is horrific. Iran must take full responsibility," Canadian Prime Minister Justin Trudeau, whose country had 57 citizens on board, told a news conference in Ottawa. "Canada will not rest until we get the accountability, justice, and closure that the families deserve." Trudeau said Rouhani committed to collaborating with Canadian investigators, working to de-escalate tensions in the region and continuing a dialogue.

Up to 1,000 protesters chanted slogans in Teheran against the authorities, the semi-official Fars news agency said in a rare report on anti-government unrest.

Demonstrators ripped up pictures of Qassem Soleimani, a prominent Iranian military commander who was killed in a Jan 3 by a US drone strike in Iraq ordered by Trump.

Iranian missile strikes on US targets in Iraq on Wednesday in retaliation for the killing led to Iran being on a state of high alert for possible reprisals when the plane was downed.

On Twitter, videos showed protesters demanding that Khamenei step down because of the disaster.

"Commander-in-chief resign, resign," hundreds chanted in front of Teheran's Amir Kabir university. Reuters could not verify the footage.

In a televised address on Saturday, Ukrainian President Volodymyr Zelensky said he had agreed with Rouhani on the beginning of joint work on decoding the black boxes from the downed aircraft. Zelensky also urged Ukraine's international partners to be united and persistent until the investigation was complete.

Earlier, Zelensky said on Twitter Iran's acknowledgement was a step in the right direction but added, "The perpetrators must be held accountable."

Zelensky said Rouhani had apologised to him on behalf of Iran.

Khamenei, until now silent about the crash, said information about the incident should be made public.

The shooting down of the passenger jet heightened international pressure on Iran after months of friction with the United States and tit-for-tat attacks. Canada and the United States had both said early on that they believed an Iranian missile brought down the aircraft, probably in error.

"The Islamic Republic of Iran deeply regrets this disastrous mistake," Rouhani wrote on Twitter, promising that those responsible would be prosecuted. "My thoughts and prayers go to all the mourning families."

US Secretary of State Mike Pompeo posted on Twitter a video of the protests in Teheran with the caption, "The voice of the Iranian people is clear. They are fed up with the regime's lies, corruption, ineptitude and brutality" of the Islamic Revolutionary Guard Corps, a branch of the Iranian military, under what he called Khamenei's "kleptocracy."

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

In Twitter messages, angry Iranians asked why the plane was allowed to take off with tensions in Iran so high.

The plane, a Boeing 737-800 en route for Kiev, came down shortly after take-off from Teheran, when Iran was alert for US reprisals after launching rockets at US troops in Iraqi bases.

Iran's Revolutionary Guards, in a rare step, apologised to the nation and accepted full responsibility. Senior Guards commander Amirali 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.

Iranian Foreign 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 Revolutionary Guards site at a time of high alert.

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

Ukraine International Airlines said Iran should have closed the airport. The carrier said it had received no indication it faced a threat and was cleared for take off.