TEHRAN (AFP) - School bells rang out 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 now mired in a political crisis. Reformist ex-president Mohammad Khatami, now a strong backer of anti-government protesters, urged his supporters to join the February 11 march that marks the toppling of the US-backed shah 31 years ago. Mondays nationwide clamour, at precisely 9:33 am (0603 GMT), marked the moment Khomeinis chartered Air France 747 touched down at Tehrans Mehrabad airport on February 1, 1979. President Nejad and other senior officials paid homage to the cleric at his gold-domed mausoleum in southern Tehran. The celebrations that began on Monday climax on February 11, the anniversary of the fall of shah who had ruled Iran for nearly four decades, but who fled two weeks before Khomeinis return. The charismatic cleric assumed the role of the Islamic republics supreme leader, a post he held until his death in 1989. During his conservative rule, Iran became embroiled in a brutal conflict with Saddam Husseins neighbouring Iraq. More than a million people were killed on both sides during the war. Khomeini backed students who stormed the American embassy in the capital in November 1979 and took its personnel hostage for 444 days, prompting Washington-branded the Great Satan by Iran-to sever ties with Tehran. Relations between Iran and the United States have slid steadily downhill during the rule of hardliner Ahmadinejad, who was controversially re-elected last June in an election the opposition claims was massively rigged. Tension has also peaked between Iran and the West over Tehrans nuclear programme, which world powers believe is masking an atomic weapons programme 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 Khomeinis mausoleum. In a speech attended mostly by military personnel and relatives of those killed in the war with Iraq at the Behesht Zahra cemetery south of Tehran city, ex-parliament speaker Gholam Ali Haddad Adel insisted that Iranians are sticking to the commitment made 31 years ago. They 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 The backdrop of this years anniversary is Ahmadinejads re-election, which triggered one of the worst political crises in the history of the Islamic republic. Some early pillars of the regime including Mir Hossein Mousavi-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 Khatami have also backed opposition groups, in turn shaking the regimes foundations and bitterly dividing the clergy. Violent clashes since the election between opposition supporters and security forces have killed dozens, wounded hundreds and seen thousands of arrests. Hundreds of people have been put on trial accused of trying to topple the regime. Iran executed two dissidents on Thursday for their alleged role in post-election violence. The 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 that day, Karroubis website Sahamnews.org said on Saturday, while Khatami urged his supporters on Monday to join the annual march. This year hopefully people from all persuasions and positions will mark the 31st anniversary of the revolution, he said on the website of his organisation, Baran. We are also the defenders of the revolution and if we have complaints let us talk. Civil protest should not be met with pressure, suppression, jail or even execution, he said. People have the right to protest.