The overthrow of the Shah came as a surprise to almost all observers. The first militant anti-Shah demonstrations of a few hundred started in October 1977, after the death of Khomeini's son Mostafa. A year later strikes were paralysing the country, and in early December a "total of 6 to 9 million" marched against the Shah throughout Iran.
(https://en.wikipedia.org/wiki/Mohammad_Reza_Pahlavi)