About a thousand people gathered outside the Diyarbakir municipality to demand the release of mayor Gultan Kisanak and co-mayor Firat Anli, who were taken into custody late Tuesday as part of a terrorism investigation, AP reported.
Earlier Wednesday, top European Union officials voiced concern over the detentions, calling on Turkey to respect the rule of law even as it deals with the security threat posed by Kurdish militants.
In a statement, EU foreign policy chief Federica Mogherini and Commissioner Johannes Hahn called on Turkey to resume Kurdish peace efforts, saying "a political solution is the only viable option."
In Diyarbakir, police cordoned off the municipality building after the governor's office issued a written statement calling for "common sense" and reiterated a ban on open-air gatherings that has been in effect since August under Turkey's state of emergency.
Turkey declared a state of emergency following the failed coup in July, allowing the government to rule by decrees and authorities to detain suspects for up to 30 days.
Kisanak and Anli were detained for questioning over alleged links to the rebels of the Kurdistan Workers' Party, or PKK. They are suspected of "speaking positively about the terror organization" and using municipality vehicles for Kurdish militants' funerals, according to a statement by the province's prosecutor's office.