Muzaffar ad Din (the shah from 1896 to 1907), was a weak and ineffectual ruler. Royal extravagance and the absence of incoming revenues exacerbated financial problems. The shah quickly spent two large loans from Russia, partly on trips to Europe. Public anger fed on the shah's propensity for granting concessions to Europeans in return for generous payments to him and his officials. People began to demand a curb on royal authority and the establishment of the rule of law as their concern over foreign, and especially Russian, influence grew.

The shah's failure to respond to protests by the religious establishment, the merchants, and other classes led the merchants and clerical leaders in January 1906 to take sanctuary from probable arrest in mosques in Tehran and outside the capital. When the shah reneged on a promise to permit the establishment of a "house of justice," or consultative assembly, 10,000 people, led by the merchants, took sanctuary in June in the compound of the British legation in Tehran. In August the shah was forced to issue a decree promising a constitution. In October an elected assembly convened and drew up a constitution that provided for strict limitations on royal power, an elected parliament, or Majlis, with wide powers to represent the people, and a government with a cabinet subject to confirmation by the Majlis. The shah signed the constitution on December 30, 1906. He died five days later. The Supplementary Fundamental Laws approved in 1907 provided, within limits, for freedom of press, speech, and association, and for security of life and property. According to scholar Ann K.S. Lambton, the Constitutional Revolution marked the end of the medieval period in Iran. The hopes for constitutional rule were not realized, however.

Muzaffar ad Din's successor, Mohammad Ali Shah, was determined to crush the constitution.