A group of Iranian protesters demanded Saturday that the country's supreme leader, Ayatollah Ali Khamenei, step down after the government said its military had by mistake shot down a Ukrainian plane, killing all 176 people on board.

Also on Saturday, Iran briefly detained the British ambassador to Iran.

British Foreign Secretary Dominic Raab slammed the Iranian action as breaking international law.

"The arrest of our Ambassador in Tehran without grounds or explanation is a flagrant violation of international law," Raab said. "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."

The protesters in Iran on Saturday expressed anger at what they perceived as an initial cover-up on the cause of the crash of Ukraine International Airlines Flight PS752, which went down shortly after it took off from Tehran early Wednesday.

Prior to Iran's admission Saturday, it had denied shooting down the plane.

Those aboard the doomed flight included 82 Iranians, 63 Canadians and 11 Ukrainians, including the nine crew members. Ten passengers were from Sweden, four from Afghanistan, three from Germany and three from the United Kingdom.

U.S. intelligence officials had said earlier that evidence suggested the plane was shot down by an Iranian missile by mistake, multiple officials said.

"Commander-in-chief (Khamenei) resign, resign," videos posted on Twitter showed hundreds of people chanting, in front of Tehran’s Amir Kabir university.

The crowds included middle-class professionals, as well as students.

President Donald Trump weighed in Saturday, tweeting support for the protesters.