Iranian President Ebrahim Raisi vowed on Tuesday to avenge the killing of a top general three years ago in a US drone strike.
The assassination of Gen Qassem Suleimani, 62, on January 3, 2020, was ordered by then-US president Donald Trump.
Suleimani commanded the Quds Force, the foreign operations arm of the Islamic Revolutionary Guard Corps, and has been lionised by Iranian authorities.
He was one of the country's most popular public figures, who led Iran's Middle East operations and was seen as a hero of the 1980-1988 Iran-Iraq war.
"We have not forgotten and will not forget the blood of martyr Suleimani," Mr Raisi said during a commemoration at Tehran's Grand Mosalla mosque, where thousands waved Iranian flags and held pictures of the dead commander.
The "murderers and perpetrators" of his killing should know that "revenge for the blood of martyr Suleimani is certain and they will not sleep easily", he said.
Iraqis march to commemorate Qassem Suleimani - in pictures
Suleimani died with his Iraqi lieutenant Abu Mahdi Al Muhandis in the strike near Baghdad airport.
Days later, Iran retaliated by firing missiles at bases in neighbouring Iraq housing American and other coalition troops.
No US personnel were killed but Washington said dozens suffered traumatic brain injuries from the blasts.
Amid the heightened tension, Iran also accidentally downed a Ukrainian passenger jet on January 8, 2020, killing all 176 people aboard.
Iranian minister in Baghdad visits site where Qassem Suleimani was killed - video
Iranian minister in Baghdad visits site where Qassem Suleimani was killed
The Pentagon said at the time that Suleimani had been "actively developing plans to attack American diplomats and service members in Iraq and throughout the region".
But "none of Mr Trump's objectives" were achieved by the killing of Mr Suleimani and his assistant, the head of the Iran-backed Lebanese militant group Hezbollah, Hassan Nasrallah, told his followers at a commemorative event.
"Our axis got stronger with their blessed blood, the axis of resistance that we must strongly preserve and maintain," Nasrallah said.
