Iran on Friday began celebrating the 40th anniversary of the country's 1979 Islamic Revolution, which toppled the US-backed shah, overturned 2,500 years of monarchical rule and brought hardline Shiite clerics to power.
The climactic events that year in Iran, when images of revolutionaries in the streets gave way to black-and-white images of blindfolded American hostages in the US Embassy crisis months later, not only changed Iran's history but also helped shape today's Middle East.
The anniversary starts every year on February 1, the day Ayatollah Ruhollah Khomeini returned home from France, after 14 years in exile, to become the supreme leader of the Islamic Republic of Iran.
Across Iran, sirens rang out from trains and boats and church bells chimed at 9.33 am on Friday, the exact time Khomeini's chartered Air France Boeing 747 touched down 40 years ago at Tehran's Mehrabad International Airport.
The 10-day anniversary festivities, known as the Ten Days of Dawn, end on February 11, the date Shah Mohammad Reza Pahlavi's government collapsed after brief clashes between some units of the army and revolutionary gunmen and following nationwide protests.
As part of the celebrations, many buildings in Tehran, mostly government institutions and offices, were draped in the colours of Iran's green-white-and-red flag while multi-coloured lights decorated the main streets.
In memory of those events, car drivers turned on their headlights and honked in celebration as helicopters dropped clusters of flowers along the 21-mile route from the airport to the Behesht-e-Zahra cemetery in southern Tehran, where Khomeini made his first speech and where his tomb stands today.
Iranian officials, including Supreme Leader Ayatollah Ali Khamenei and President Hassan Rouhani, on Wednesday paid homage to Khomeini's tomb "to renew allegiance" to the late leader.
Iranian state TV broadcast archive footage of Khomeini's return and the daily mass demonstrations across Iran in support to his revolution.
Khomeini was accompanied on the flight home by dozens of journalists, a few of his associates and only one family member, his younger son Ahmad.
The plane was only half full, to keep room for extra fuel in case of immediate return to Paris if the plane could not land in Tehran.
Supporters of the shah's regime had closed the airport the week before and Khomeini's allies in Tehran feared possible threats against his life.