After charging the rally in the central square of Diyarbakir, police armed with automatic rifles forced scores of people to kneel on the ground. Other police searched nearby coffeehouses and billiard halls for more activists.
Diyarbakir is the main city in southeast Turkey, where Kurdish rebels have been fighting for autonomy since 1984. More than 37,000 people have been killed in the conflict. Turkish security forces have clamped down on the area amid rioting and violence sparked by the February arrest of Kurdish rebel leader Abdullah Ocalan.
Tuesday's gathering was the first election rally in Diyarbakir for the pro-Kurdish People's Democracy Party, or HADEP, which is expected to sweep local elections April 18th.
Police canceled the rally just 25 minutes before it was to begin, leaving no time for party officials to tell supporters not to come, said acting party president Osman Ozcelik.
Officers armed with clubs charged into the square, swinging batons to disperse hundreds of people who had already gathered. Armored police cars and trucks of soldiers rushed in to help.
Police Chief Ali Gabbar Okan, citing security reasons for the rally's cancellation, said around 300 people were detained.
Ozcelik, however, said several thousand people were detained in the morning on the way to the rally.
HADEP calls for cultural rights for Turkey's Kurdish minority, such as the right to use the Kurdish language in schools and on television.
Prosecutors claim it is linked to Ocalan's rebel Kurdistan Workers Party and are trying to shut it down.
Police have detained hundreds of party activists since Ocalan was arrested in February. More than ten party offices have been closed on charges of possessing illegal books or magazines.