Turkish police detained 90 people on Tuesday over suspected links to Kurdistan Workers' Party (PKK) rebels, the government said.
Turkey's main pro-Kurdish party condemned the arrests, calling it a politically motivated crackdown.
The operation across eight provinces is continuing, the Interior Ministry said.
Tuesday's arrests come days after President Recep Tayyip Erdogan warned he would replace any mayors elected in next year's local elections if they were thought to have links to terrorism.
Mr Erdogan accuses the main pro-Kurdish party, the Peoples' Democratic Party (HDP), of links to the PKK. The HDP denies this and says it is unjustly targeted by the government.
More than 140 HDP members were detained over several days, the party said on Sunday.
Ninety four of 102 municipalities in Kurdish-majority cities and towns are now administered by trustees, rather than their elected mayors. The authorities removed the mayors, elected in Turkey's last municipal elections in 2014, in a security crackdown that followed an July 2016's attempted military coup.
"Elections are nearing," Mr Erdogan told a meeting of his Justice and Development Party (AKP) at the weekend. "If those involved with terror come out of the ballot box, we shall appoint trustees without delay."
One prominent HDP lawmaker, Garo Paylan, said the arrests were politically motivated and were part of the AKP's campaign strategy for the 2019 election.
"The AK Party has started its local election campaign from Diyarbakir by detaining journalists, politicians and theologians," Mr Paylan said. Diyarbakir is the largest city in the mainly Kurdish southeast.
The PKK – deemed a terrorist organisation by Turkey, the United States and Europe – has waged an insurgency since 1984. Violence across the southeast escalated after the collapse of a ceasefire in 2015.
In recent months Turkey launched regular strikes on PKK bases in northern Iraq, especially the insurgents' stronghold in the Qandil mountains, where Ankara has also threatened to carry out a ground offensive.
On Tuesday, the PKK killed a Turkish soldier and wounded four others in a missile attack launched from northern Iraq on a military post in Cukurca, in Turkey's south-eastern Hakkari province.