Iran’s security forces deployed in large numbers across the capital on Sunday, expecting more 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 in black uniforms and helmets massed in Vali-e Asr Square, Tehran University and other landmarks as calls circulated for protests later in the day. 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 the police, hoping not to draw attention to themselves.

The plane crash early Wednesday killed all 176 people 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 finally admitted to accidentally shooting it down in the face of mounting evidence and accusations by Western leaders.

Iran downed the Ukrainian flight as it braced for retaliation after firing ballistic missiles at two bases in Iraq housing U.S. 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 U.S. 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. They are also mourning the dead, among whom were a large number of young people with promising futures 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 only identified himself as Saeed, said the 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.

Hundreds of students gathered at Tehran’s Shahid Beheshti University on Sunday to mourn the victims and protest against authorities for concealing the cause of the crash, the semi-official ISNA news agency reported. They later dispersed peacefully.

Others, including Foreign Minister Mohammad Javad Zarif, posted all-black photos on their Instagram accounts in mourning.

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 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.”

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,” Foreign Secretary Dominic Raab said in a statement.

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” and calling for the ambassador to be expelled and for the closure of the embassy. Police stood guard outside the facility.

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.”

Mehdi Karroubi, an opposition activist under house arrest, lashed out at Khamenei himself.

“You, as the commander in chief of the armed forces, are directly responsible for this,” he said in a statement. “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.”

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 President Donald Trump declined to respond and welcomed Iran’s apparent decision to stand down.

The emir of Qatar, Sheikh Tamim bin Hamad Al Thani, traveled to Iran for talks with senior officials. Qatar hosts a large U.S. military base and shares an offshore gas field with Iran. It has often served as a mediator between the two countries, which have no diplomatic relations.

Syria’s Prime Minister Imad Khamis was also leading a high-level delegation to Iran, which includes the defense 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.