Forty years ago, Ayatollah Ruhollah Khomeini spearheaded an Islamic Revolution that deposed Iran's shah. The country's authoritarian monarchy was replaced by a repressive theocracy, and almost overnight, Iran was transformed from a crucial U.S. ally to bitter foe.
In Pictures: Iran's 1979 Islamic Revolution

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Tehran was crippled by strikes and mass protest against the Shah on September 7, 1978. The day became known as Black Friday and saw demonstrations spread across the country with protesters accusing the Shah of being a U.S. puppet. Martial law was imposed four days later.

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Despite the declaration of martial law, thousands of defiant demonstrators continued to flood the streets of Tehran on October 9, 1978, to protest against Iran's Shah, Mohammad Reza Pahlavi.

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Protesters set fires on the streets of the Iranian capital during antigovernment demonstrations on November 5, 1978. Protests turned deadly later that month when armed soldiers clashed with students at the University of Tehran.

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Iranian students shout "Death to the Shah" amid protests in Tehran on November 28, 1978. Mohammad Reza Shah Pahlavi had ruled Iran since 1941. While he modernized the country, he forcefully suppressed dissent and political freedoms.