Iran deployed riot police in the capital on Sunday expecting possible protests after a candlelight ceremony late on Saturday turned violent, with police dispersing hundreds of people with tear gas. Riot police and plain clothes officers could be seen massing in Vali-e Asr Square in Tehran as calls circulated for protests later in the day.

At a student protest to pay tribute to the crash victims on Saturday, Iranian authorities briefly detained Britain’s ambassador, in what the British government called a violation of international law. He was later released.

Iran’s Mehr news agency said the ambassador, Rob Macaire, was arrested for his alleged “involvement in provoking suspicious acts”.

Macaire tweeted on Sunday that he had attended what was advertised as a vigil for the victims. “Normal to want to pay respects – some of victims were British. I left after 5 mins, when some started chanting,” he said. “Arresting diplomats is of course illegal, in all countries,” he added.

Trump told Iranians – in tweets in both English and Farsi – that he stands by them and is monitoring the demonstrations.

“To the brave, long-suffering people of Iran: I’ve stood with you since the beginning of my Presidency, and my Administration will continue to stand with you,” he tweeted.

“There can not be another massacre of peaceful protesters, nor an internet shutdown. The world is watching,” he added, apparently referring to an Iranian crackdown on street protests that broke out in November.

“We are following your protests closely, and are inspired by your courage,” he said.

The new demonstrations follow an Iranian crackdown on street protests that broke out in November. Amnesty International has said it left more than 300 people dead.

Internet access was reportedly cut off in multiple Iranian provinces ahead of memorials planned a month after the protests.

Canadian Prime Minister Justin Trudeau has demanded that Iran provide “full clarity” on the downing of the plane. Ottawa says the dead included 57 Canadians.

Iran’s Supreme Leader Ayatollah Ali Khamenei also offered his condolences and ordered the armed forces to address “shortcomings” so that such a disaster does not happen again.

Tehran’s acknowledgement came after officials in Iran denied for days Western claims that the Ukraine International Airlines plane had been struck by a missile in a catastrophic error.

The Kiev-bound jet slammed into a field shortly after taking off from Tehran’s Imam Khomeini International Airport on Wednesday.

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The crash came hours after Tehran launched missiles at bases hosting American forces in Iraq in response to the killing of top Iranian general Qassem Soleimani in a US drone strike.

Fears grew of an all-out war between Iran and its arch-enemy the United States, but those concerns have subsided after Trump said Tehran appeared to be standing down after targeting the US bases.

On Saturday evening, police dispersed students who had converged on Amir Kabir University in Tehran to pay tribute to the victims, after some among the hundreds gathered shouted “destructive” slogans, Fars news agency said.

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State television reported that students shouted “anti-regime” chants, while Fars reported that posters of Soleimani had been torn down.

The aerospace commander of Iran’s Revolutionary Guards accepted full responsibility for Wednesday’s accident.

But Brigadier General Amirali Hajizadeh said the missile operator acted independently, targeting the 737 after mistaking it for a “cruise missile”.

The operator failed to obtain approval from his superiors because of disruptions to a communications system, he said.

“He had 10 seconds to decide. He could have decided to strike or not to strike and under such circumstances, he took the wrong decision.”

Iran had been under mounting international pressure to allow a “credible” investigation after video emerged appearing to show the moment the airliner was hit.

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In footage that The New York Times said it had verified, a fast-moving object is seen rising into the sky before a bright flash appears. Several seconds later, an explosion is heard.

Most passengers on Flight PS752 were Iranians and Canadians, including dual nationals, while Ukrainians, Afghans, Britons and Swedes were also aboard.

Rowhani told his Ukrainian counterpart Saturday that “all the persons involved in this air disaster will be brought to justice,” Ukraine’s presidency said.

This is Iran’s worst civil aviation disaster since 1988 when the US military said it shot down an Iran Air plane over the Gulf by mistake, killing all 290 people on board.

Additional reporting by Associated Press and DPA