Leader of the Islamic Revolution Ayatollah Seyyed Ali Khamenei is due to deliver a speech at the event later in the day.
Some 50 foreign correspondents and 300 Iranian reporters will be covering the event.
Grand Ayatollah Seyyed Rouhollah Mousavi Khomeini, better known as Imam Khomeini, passed away on June 3, 1989 at the age of 87.
He contributed many years of his life to standing up to the US-backed Pahlavi dynasty, and eventually paved the way for its downfall in the 1979 Islamic Revolution.
Pilgrims visit the Mausoleum of Imam Khomeini, the late founder of the Islamic Republic, on the occasion of 40th anniversary of his return from exile, on February 01, 2019. (Photo by AFP)
In the pre-Revolution era, Imam Khomeini spent more than 15 years in exile for his opposition to the last monarch, Mohammad Reza Pahlavi, mostly for his association with Western imperialists.
He was not allowed to return to Iran during Pahlavi’s reign, and only came back home on February 1, 1979 after the monarch finally gave into angry popular demonstrations and fled the country. The tyrannical Pahlavi regime fully collapsed 10 days later on February 11.
The mourning ceremonies precede the anniversary of demonstrations of June 5, 1963, which is remembered in Iran as a prelude to the victory of the Islamic Revolution.
Prior to the Revolution, Imam Khomeni was arrested in 1963 after he made a historic speech in the holy city of Qom, where he lambasted the “capitulation law” granting immunity to Americans on Iranian soil.
Later that year, people took to the streets to protest the political leader’s arrest. Taken by surprise by the massive public demonstrations of support, the Pahlavi regime’s forces launched a bloody crackdown on people to quell the protests.