Timeline: Iran's road to presidential elections

(Reuters) - Iranians vote in a presidential election on Friday in an election that pits President Mahmoud Ahmadinejad against two moderate challengers and a fellow-conservative.

Iran, the world's fifth biggest oil exporter, is grappling with the impact of the global economic downturn and weighing its response to U.S. President Barack Obama's diplomatic overtures.

Here is a timeline of the main political events since the 1979 revolution and founding of the Islamic Republic:

January 1979 - Shah Mohammad Reza Pahlavi is forced into exile after mounting discontent with his authoritarian rule.

February 1979 - Ayatollah Ruhollah Khomeini returns to Tehran from Paris after 15 years of exile spent mainly in Iraq and quickly consolidates his power to become Supreme Leader.

June 1989 - Khomeini dies. Ayatollah Ali Khamenei, a former president, is appointed Supreme Leader.

August 1989 - Pragmatic deal-maker Akbar Hashemi Rafsanjani becomes president after landslide election win.

May 1997 - Reformist Mohammad Khatami is elected president in surprise and sweeping victory over candidate backed by conservative clerics.

July 1999 - Student unrest turns violent over closure of pro-reform newspaper. Dozens of such newspapers are later shut.

February 2000 - Khatami's pro-reform allies secure a big win in parliamentary election against entrenched conservatives.

June 2001 - Khatami is re-elected president.

August 2002 - Exiled opposition National Council of Resistance of Iran reports existence of uranium enrichment facility at Natanz and heavy water plant at Arak.

February 2004 - Conservatives win parliamentary poll after Guardian Council bars some 2,500 reformers from standing.

June 2005 - In major upset, Mahmoud Ahmadinejad, mayor of Tehran, defeats Rafsanjani in run-off vote for presidency.

May 2006 - Washington offers to join multilateral talks with Iran if it verifiably suspends uranium enrichment.

December 2007 - A U.S. National Intelligence Estimate report says Iran put its bid to build a nuclear bomb on hold in 2003.

February 2008 - Hardline state bodies bar hundreds of reformists from running in March 14 parliamentary election.

March 2008 - U.N. Security Council adopts third sanctions resolution targeted at Iran's nuclear programme.

November 2008 - Iran says it is running 5,000 uranium enrichment centrifuges, signaling an expansion of work the West fears is aimed at making nuclear weapons.

January 2009 - After three decades of mutual mistrust, new U.S. President Barack Obama offers a new beginning of engagement with Tehran if it "unclenches its fist."

February 2009 - A new IAEA report shows a significant increase in Iran's reported stockpile of low-enriched uranium since November to 1,010 kg, enough, some physicists say, for possible conversion into high-enriched uranium for one bomb.

March 2009 - Obama says he is to extend sanctions against Iran as it continued to pose an "extraordinary threat" to the United States. Later in March Obama issues an unprecedented videotaped appeal to Iran offering a "new beginning" of diplomatic engagement.

-- Supreme Leader Ayatollah Ali Khamenei, a day after the videotape says Obama's offer of better ties is just a "slogan," but pledges Tehran will respond to any real policy shift by Washington adding: "You change, our behavior will change."

April 2009 - Ahmadinejad accuses Israel of mass murder and ethnic cleansing against the Palestinians, two days after his denunciation of the Jewish state as racist prompted a walk-out from a U.N. meeting on race.

June 2009 - Presidential elections.