Thousands of Iranians have gathered to honour the memory of former Supreme Leader Ali Khamenei, who governed the nation for nearly 40 years until he was killed in US-Israeli strikes at the beginning of the latest Middle East conflict.

His successor, son Mojtaba Khamenei, has remained out of public view since before the war began, and was not expected to attend the nationwide commemorative events held Thursday.

State television broadcast footage showing thousands participating in memorial rallies across Iran on Thursday, with many carrying portraits of the late leader in cities including Urmia in the northwest, Gorgan in the northeast, and the capital Tehran.

The national tribute commenced at 9:40am (06:10 GMT), the exact time Ali Khamenei was killed on February 28 at his Tehran residence, along with several other senior Iranian officials.

The attack initiated the Middle East conflict that has since engulfed the region, with Iran responding by targeting US assets and allies across the Gulf and launching strikes against Israel.

Due to the ongoing war, a formal state funeral for the late supreme leader has yet to be held.