The joint leaders of Turkey's pro-Kurdish opposition party, People's Democracy (HDP), have been arrested along with at least nine other MPs.
Selahattin Demirtas and Figen Yuksekdag are accused of spreading propaganda for militants fighting the Turkish state.
Hours after Mr Demirtas was arrested in Diyarbakir, a car bomb killed eight people and injured more than 100.
Militants have been fighting for years to achieve independence for the Kurds, Turkey's biggest ethnic minority.
Turkey remains under a state of emergency that was imposed after a failed military coup in July.
The emergency allows President Recep Tayyip Erdogan and his cabinet to bypass parliament when drafting new laws and to restrict or suspend rights and freedoms.
A major escalation: Analysis by Mark Lowen, BBC Turkey correspondent
The government says those detained had failed to respond to a summons for questioning and issued an arrest warrant for two other HDP MPs currently abroad.
This is a major escalation of a clampdown that has seen Kurdish media closed down and the mayor of Diyarbakir arrested.
Hopes of an end to Turkey's decades-long Kurdish problem have evaporated since a ceasefire with the [Kurdistan Workers' Party] PKK broke down in 2015, leading to a wave of tit-for-tat attacks.
Mr Demirtas elicited international support with his liberal politics but critics say he has failed to distance the party sufficiently from the PKK. Friday's detentions are likely to provoke more tension among Kurds - and more violence too.
Why were the party leaders arrested?
The government says they were detained for failing to co-operate with a counter-terrorism investigation, which the two leaders vowed to boycott in June.
Defending the arrests, Justice Minister Bekir Bozdag said: "They did not respect the law."
The MPs are also accused of spreading propaganda for the PKK, the Kurdish group suspected of a wave of recent attacks including Friday's car bomb.
Deemed a terrorist organisation by the US, the EU and Turkey, the PKK has been fighting the state since the 1970s in a war which has claimed at least 40,000 lives.
Can MPs really be arrested just like this in Turkey?
Turkish politicians normally have immunity from prosecution but this was removed from the HDP and some other MPs in May.
Last month, the joint mayors of Diyarbakir, Turkey's largest Kurdish-majority city, were also arrested as part of a terrorism investigation.
Is the HDP linked to the PKK?
While commonly seen as the main pro-Kurdish party, HDP is an eclectic grouping which also appeals to leftists, liberals, environmentalists, gay rights activists and pious Muslims. That explains why it is Turkey's third-largest party.
It entered parliament for the first time last year, winning 59 seats.
During the election campaign, it carried a moderate message despite dozens of attacks against party supporters and offices, culminating in a deadly attack on its largest election rally in Diyarbakir two days before the vote.
The party strongly denies any links to the PKK.
Mr Demirtas and others accuse President Erdogan of seeking to push the party out of parliament and increase his own power.
One MP, who is currently abroad, told the BBC the government was acting like Nazis.
"This crackdown tonight is nothing to do with procedural law, criminal law, any law whatsoever or the constitution," Ertugrul Kurkcu said. "This is an unlawful hijacking of HDP parliamentarians."