The ceremonies for the 1979 revolution started at 9:33 am (0603 GMT), the exact time that Khomeini returned to Iran and his plane touched down at Tehran airport.
As an army band played revolutionary anthems, the huge hall of the mausoleum was filled with people from all walks of life, amid schoolchildren dressed in the red, white and green of the Iranian flag.
Ayatollah Ahmad Jannati, head of the influential experts assembly, delivered a keynote speech rebuking factions seeking better ties with Washington.
"Curses on the wrong school of thought that thinks we can't run the country unless America helps us," he said, shaking a raised left hand.
"America's power is on the decline, we should not be afraid of America," Jannati said.
Iranian people have started the 10-Day Dawn celebrations to mark the 40th anniversary of the country’s 1979 Islamic Revolution, which overthrew the monarchy of the US-backed Pahlavi regime in Iran.
A special ceremony is being held at Imam Khomeini’s mausoleum south of Tehran, with a host of senior state and military officials as well as thousands of people from all walks of life in attendance.
Another ceremony is held at Behesht-e Zahra cemetery south of the capital, where Imam Khomeini delivered his first speech after the exile, in an address to five million people.
Similar ceremonies are held across Iran and in other parts of the world to mark the first day of the Ten-Day Dawn period.
Imam Khomeini spent more than 14 years in exile, mostly in the Iraqi holy city of Najaf. He also spent some time in Turkey and France before his return to Iran.
Millions of people converged on the capital from across the country on the day of his return. His arrival gave considerable momentum to popular protests against the US-backed Pahlavi regime, which eventually led to its overthrow ten days later.
The 10-Day Dawn festivities will culminate in nationwide rallies on February 10 this year, the anniversary of the Islamic Revolution.
The period witnessed last-ditch efforts by the Washington-backed regime of Iran’s former monarch to hold onto power by unleashing deadly force against pro-Revolution protesters.
The entire struggle resulting in the Revolution was led by Imam Khomeini, who successfully united the people against Pahlavi oppression and dependence on the US.