Co-leaders of the pro-Kurdish HDP, Selahattin Demirtas and Figen Yuksekdag, arrested on terrorism-related charges.
Turkish police have detained more than 800 people over alleged links to Kurdish fighters in nationwide operations, according to state media.
Police conducted simultaneous raids in 37 different provinces and taken 834 people into custody, state-run Anadolu Agency quoted police forces as saying on Tuesday.
Anadolu said authorities received intelligence that the outlawed Kurdistan Workers’ Party (PKK) would escalate attacks after February 15, the day marking the 1999 capture of imprisoned PKK leader, Abdullah Ocalan.
Along with the suspects, two Kalashnikovs, 11 guns, 15 rifles and ammunition were seized, police said.
The operations come as Turks prepare to vote on April 16 on replacing the parliamentary system with the stronger presidency sought by President Recep Tayyip Erdogan.
The referendum will be held under a state of emergency imposed after an attempted coup last July.
Tens of thousands of people have been arrested since the abortive coup over suspected ties to Fethullah Gulen, the US-based Muslim leader and businessman accused by Ankara of organising the coup.
The pro-Kurdish Peoples’ Democratic Party (HDP) said the raids aimed to weaken it.
“The basic goal of these operations … is to hold the referendum without the HDP,” a statement from its executive committee said.
Its statement, released before Tuesday’s arrests, said more than 300 of its members and executives had been detained on Monday, bringing those held this year to around 1,200.
A dozen of its MPs and tens of Kurdish mayors from a sister party have been jailed pending trial.
The government accuses the HDP, parliament’s second biggest opposition party, of being a political extension of the PKK.
The HDP denies direct links with the PKK and says it wants a peaceful settlement in Turkey’s largely Kurdish southeast.
The PKK, designated a “terrorist” group by Turkey, the EU and the US, launched an armed struggle against the Turkish state in 1984 and more than 40,000 people have been killed in the conflict.
A ceasefire between the PKK and the state broke down in July 2015 and thousands have been killed in the conflict since then.