Thousands of people chanted against Israel and the United States at Friday's funeral for seven Islamic Revolutionary Guard Corps members killed in a strike in Syria, which Iran blamed on Israel.
The IRGC members, including two generals, were killed in the air strike on Monday which levelled the Iranian embassy's consular annex in Damascus.
IRGC chief General Hossein Salami in a speech said that Israel "cannot escape the consequences".
Israel has not commented on the strike, but analysts saw it as an escalation of its campaign against Iran and its regional proxies that runs the risk of triggering a wider war beyond the Israel-Hamas conflict in the Gaza Strip.
Friday's ceremony coincides with the annual Quds (Jerusalem) Day commemorations, when Iran and its allies stage marches in support of the Palestinians.
Quds rallies were also planned for other countries on Friday, including Syria and Lebanon.
Iran has said among the dead were two brigadier generals from the Guards' foreign operations arm, the Quds Force, Mohammad Hadi Haji Rahimi and Mohammad Reza Zahedi.
A Britain-based war monitor, the Syrian Observatory for Human Rights, said Zahedi was the Quds Force commander for the Palestinian Territories, Syria and Lebanon.
The 63-year-old had held a succession of commands in a Guards career spanning more than 40 years, and was the most important Iranian soldier killed since a United States missile strike at Baghdad airport in 2020 killed General Qassem Soleimani, who headed the Quds Force.
The coffins of the seven were placed on the trailers of two trucks in one of the largest squares in Tehran.
Mourners held Iranian and Palestinian flags, as well as those from the Iran-backed Lebanese Hezbollah group.
"Death to Israel!" and "Death to America!" they chanted.
Israel 'will be punished'
The Fars news agency reported that among those present was Ziyad al-Nakhalah, leader of the Islamic Jihad group that fights alongside Hamas in Gaza.
President Ebrahim Raisi and his predecessor Hassan Rouhani also attended, local media reported.
In his speech, IRGC chief Salami reaffirmed Tehran's determination to punish Israel for the Damascus raid.
"The Zionist regime cannot escape the consequences of the harm it does," he said. "It is exposed and knows very well what is going to happen."
Pictures of the dead men were displayed on the trucks, accompanied by the slogan "Martyrs on the road to Jerusalem".
Iran's supreme leader Ayatollah Ali Khamenei has said Israel "will be punished" for the killings.
The Gaza war began with Hamas's October 7 attack on Israel, which left 1,170 people, mostly civilians, dead in southern Israel. Tehran backs Hamas, but has denied any direct involvement in that attack.
Israel has vowed to eradicate Hamas and has launched a blistering campaign against the Palestinian militants which has killed at least 33,091 people, mostly women and children, according to the Hamas-run health ministry in Gaza.
One placard on Friday echoed Khamenei's pledge to strike back after the Damascus strike: "We will make them regret this crime," it said.
State television broadcast footage of similar gatherings on Friday in other Iranian cities including Mashhad, Qom, Sanandaj and Shahrekord.
Khamenei said on Wednesday the Damascus strike was a "desperate" effort by Israel that "will not save them from defeat" in Gaza.
Israel announced on Thursday it was strengthening its defences and pausing leave for combat units following Iran's threats to retaliate.
Monday's attack in Damascus, which the Observatory said killed 16 people, was the fifth raid on Syria in a week.
Iran, Israel's arch-foe, has been a major ally of Syrian President Bashar al-Assad during his country's 13-year civil war.
Israel has long fought a shadow war of assassinations and sabotage against Iran and its armed allies, including Hezbollah and other militant groups, carrying out hundreds of strikes against targets in Syria.
The number of attacks has intensified since the Gaza war between Israel and Hamas began.
"It is quite clear that America has been one of the principal actors in the war continuing," Iranian Foreign Minister Hossein Amir-Abdollahian said in a statement published on Friday.
IRGC chief Salami in his speech also denounced Washington.
"The Zionist regime is alive and well today because of artificial support from the United States. When this is withdrawn it will collapse, and this is near," he said.
After Friday's funeral ceremony in Tehran, Zahedi will be buried in his home city of Isfahan in central Iran.