The Council of Europe, also known as the Venice Commission, said Turkish authorities had good reasons to declare a state of emergency in response to a "dangerous armed conspiracy," but concluded that Ankara's measures contravened the country's constitution and international law.
The opinion took particular issue with the collective dismissal of "tens of thousands of public servants" on the basis of lists appended to emergency decrees rather than "verifiable evidence" in each individual case.
Turkish authorities consider the Gulen movement to be a terrorist organization on par with the outlawed Kurdistan Workers' Party, or PKK, and the Islamic State group.
Gulen, who denies masterminding the coup attempt, has long lived in self-imposed exile in Pennsylvania. Turkey has asked the United States to extradite him to face justice.
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Turkish officials gave no immediate response.
The panel observed that people were fired or arrested based on "connections" to a terror group that were "too loosely defined and did not require a meaningful connection with such organizations."
Turkey has arrested over 37,000 people and dismissed or suspended more than 100,000 others in the civil service, judiciary, police, military and other institutions since the abortive putsch.
Western governments and major human rights campaigners have censured Ankara's crackdown, saying it goes beyond the rule of law.