School bells pealed as trains and ships throughout Iran sounded their horns on Monday, marking the 1979 return from exile of Ayatollah Ruhollah Khomeini, who triggered a revolution that spawned an Islamic state that is now mired in a political crisis.
The clamour, at precisely 9:33 am (0603 GMT), marked the moment that Khomeini's chartered Air France 747 touched down at Tehran's Mehrabad airport on February 1, 1979.
President Mahmoud Ahmadinejad and other senior Iranian officials paid homage to the cleric at his golden-domed mausoleum in southern Tehran.
The celebrations that kicked off Monday climax on February 11, the 31st anniversary of the fall of the US-backed shah who had ruled the country for nearly four decades, but who fled two weeks before Khomeini's return.
The charismatic cleric assumed the role of Iran's supreme leader, which he held until his death in 1989.
During his conservative rule, Iran became embroiled in a brutal conflict with Saddam Hussein's neighboring Iraq. Over a million people were killed on both sides during the war.
Khomeini had also backed Islamist students who stormed the US embassy in Tehran in November 1979 and took its personnel hostage for 444 days, prompting Washington to sever ties with the Islamic republic.
Khomeini branded the United States as the "Great Satan" and diplomatic ties were severed.
Relations between Iran and the US have deteriorated further during the rule of hardliner Ahmadinejad, who was controversially re-elected last June in a poll the opposition claims was massively rigged.
Tension has also peaked between Iran and the West over Tehran's nuclear program, which world powers believe is masking an atomic weapons program despite repeated denials by Iranian leaders.
"I am sure the Iranian people will do something on February 11, which will disappoint the enemies and the tyrants completely," Ahmadinejad said in a short address at the mausoleum.
In a speech mostly attended by military personnel and relatives of those slain in war with Iraq at the Behesht Zahra cemetery south of Tehran, ex-parliament speaker Gholam Ali Haddad Adel insisted that Iranians "are sticking to the commitment made 31 years ago."
"They (Iranians) are telling the tyrants that they are moving full steam ahead to defend the revolution," Haddad Adel said as the crowd chanted "Death to America! Death to Israel!"
Women in black chadors, and men in tie-less shirts that are the norm in Islamic Iran, waved posters of Khomeini and Iranian flags as Haddad Adel spoke from the spot where Khomeini made his arrival address.
June re-election crisis
The backdrop of this year's anniversary is Ahmadinejad's June 12 re-election, which has triggered one of the worst political crises in the history of the Islamic republic.
Some of the early pillars of the regime such as Mir Hossein Mousavi, who was prime minister under Khomeini, and former parliament speaker Mehdi Karroubi have turned into its bitter opponents, accusing the authorities of "fraudulently" re-electing Ahmadinejad.
Powerful figures such as ex-presidents Akbar Hashemi Rafsanjani and Mohammad Khatami have backed the opposition groups, in turn shaking the pillars of the regime and bitterly dividing the nation's clergy.
Violent clashes since the election between opposition supporters and security forces have killed dozens, wounded hundreds and seen thousands of arrests.
Hundreds of protesters have been put on trial for attempting to topple the regime.
Iranian authorities have warned that they will crack down on protesters if they stage another anti-government demonstration on February 11, when traditionally hundreds of thousands of Iranians take to the streets in celebration.
Mousavi and Karroubi have implicitly called on their supporters to demonstrate on February 11, Karroubi's website Sahamnews.org said on Saturday.