Then-US president Donald Trump ordered the strike which assassinated General Qassem Soleimani, 62, on January 3, 2020.

Soleimani commanded the Quds Force, the foreign operations arm of the Islamic Revolutionary Guards Corps, and has been lionised by Iranian authorities. He was one of the country's most popular public figures, who spearheaded Iran's Middle East operations and was seen as a hero of the 1980-88 Iran-Iraq war.

"We have not forgotten and will not forget the blood of martyr Soleimani," Raisi said at Tehran's Grand Mosalla mosque, where the throng waved Iranian flags and held pictures of the slain commander.

The "murderers and perpetrators" of his killing should know that "revenge for the blood of martyr Soleimani is certain and they will not sleep easily", he added.

Soleimani 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 tensions, Iran also accidentally downed a Ukrainian passenger jet on January 8, 2020, killing all 176 people aboard.

The Pentagon said at the time that Soleimani had been "actively developing plans to attack American diplomats and service members in Iraq and throughout the region."

But "none of Trump's objectives" were achieved by the killing of Soleimani 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.