British India (three modern states) had a population of 400 million at the time of its independence from Britain. That was famously a coordinated, nation-wide movement.
Indonesia was around 200 million people at the end of the Suharto dictatorship and its transition towards democracy.
British India could be good example - but there's a case to be made that it was overthrow of an external colonial rule that never integrated with the local population, so not sure there is a good parallel with the Iran situation.
Indonesia - this appears to be a really good example, along with Phillipines (1986). So what's different about Iran - why the repeated failures there?
In August of 1989, 2 million people held hands to create a chain. This was one of the large protests in human history. It led to the death of communism in Europe. More information in “The Baltic Way” article below.
Revolutions of 1989:
https://en.wikipedia.org/wiki/Revolutions_of_1989
The Baltic Way:
https://en.wikipedia.org/wiki/Baltic_Way
The Solidarity movement in Poland:
https://en.wikipedia.org/wiki/Solidarity_(Polish_trade_union...