Grand Ayatullah al-Sayyid Ruhollah Musavi Khumayni (Khomeini), a key anti-Pahlavi shah activist, oversaw the Islamization of the Iranian Revolution upon his return to the country from nearly two decades in exile. Specifically, Khumayni and his supporters oversaw the implementation of his political theory of "Islamic" governance, wilayat al-faqih (vilayat-e faqih in Persian), "guardianship of the jurist," which essentially called for a governmental system headed by the "most learned" of Iran's Twelver Shi'i religious scholars (which Khumayni was not).
With key grassroots networks in mosques and religious centers, and the backing of skilled politicians such as his former student Ayatullah Muhammad Beheshti, who headed the pro-Khumayni Islamic Republican Party, the grand ayatullah was able to successfully marginalize his clerical critics. These included the popular Ayatullah al-Sayyid Mahmoud Taliqani, who called for the establishment of a true parliamentary democracy, Grand Ayatullah al-Sayyid Muhammad Kazim Shari'atmadari, who opposed Khumaynist doctrine, and Khumayni's one-time heir apparent, Grand Ayatullah Hossein 'Ali Montazeri, after he criticized the mass execution of several thousand political dissidents and militant activists in 1988, which was approved by Khumayni.
For more background on Khumayni, see this previous post, which is based on a forthcoming encyclopedia entry I wrote.
Grand Ayatullah al-Sayyid Ruhollah Khumayni on 'Ashura (in Persian)