DUBAI, United Arab Emirates (AP) — Iran deployed riot police in the capital on Sunday expecting possible protests after its Revolutionary Guard admitted to accidentally shooting down a passenger plane at a time of soaring tensions with the United States.

Riot police and plainclothes officers could be seen massing in Vali-e Asr Square in Tehran as calls circulated for protests later in the day. A large black banner unveiled in the square bore the names of those killed in the plane crash.

The plane crash early Wednesday killed all 176 people on board, mostly Iranians and Iranian-Canadians. After initially blaming a technical failure, authorities finally admitted to accidentally shooting it down in the face of mounting evidence and accusations by Western leaders.

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The plane was shot down as Iran braced for retaliation after firing ballistic missiles at two bases in Iraq housing American forces. The ballistic missile attack, which caused no casualties, was a response to the killing of Gen. 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.

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.

Police briefly detained the British ambassador to Iran, Rob Macaire, who says he went with the intention of attending the vigil and did not know 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.”

Thanks for the many goodwill messages. Can confirm I wasn’t taking part in any demonstrations! 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. — Rob Macaire (@HMATehran) January 12, 2020

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.”

“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 deescalate tensions and engage in a diplomatic path forwards,” UK Foreign Secretary Dominic Raab said in a statement.

Iran’s Foreign Ministry said it was awaiting a police report on the arrest.

The semi-official Tasnim news agency cited an unnamed official as saying the ambassador was suspected of organizing and provoking the protesters, which it said was in violation of diplomatic protocol and justified his arrest on national security grounds.

It said he was taken to the Foreign Ministry, which later released him.

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

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

The hardline 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.”