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.
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.
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."
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 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."
Mehdi Karroubi, an opposition activist under house arrest, lashed out at Khamenei himself.
"You, as the commander of chief of the armed forces, are directly responsible for this," he said in a statement. "Please tell us, were you aware of this disaster on Wednesday morning? Or did you learn about this on Friday, as the media close to you claims?"
"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," Karroubi added.
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, meanwhile 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.