The two co-leaders of Turkey’s pro-Kurdish opposition party, People’s Democracy (HDP), have been detained along with at least nine other MPs.
Selahattin Demirtas and Figen Yuksekdag were detained at their respective homes as part of a counter-terrorism inquiry, security sources told Turkish media.
Hours after the arrest of Mr Demirtas in Diyarbakir, a suspected car bomb exploded there, injuring 20 people.
Diyarbakir, in the south-east, is Turkey’s largest Kurdish-majority city.
Ambulances could be seen rushing to the scene amid reports of at least one death as a result of the explosion, close to a police building.
Ms Yuksekdag was detained in the capital, Ankara.
Turkey remains under a state of emergency that was imposed after a failed 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.
The detention order for the MPs is for alleged propaganda for the PKK Kurdish militants, suspected of being behind Friday’s blast and a wave of recent attacks. The party strongly denies links to the group.
Police searched the HDP’s head offices in central Ankara as well as making the arrests.
A major escalation: analysis by Mark Lowen, BBC Turkey correspondent
Mr Demirtas and the rest of the party say this is an attempt by President Erdogan to push the party out of parliament and increase his own power.
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 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.
The PKK is deemed a terrorist organisation by the US, the European Union and Turkey.
Twitter, Facebook, YouTube and Whatsapp were reported to be inaccessible inside Turkey shortly after the detentions, even when users tried to circumvent restrictions using a virtual private network (VPN).
Mr Demirtas had tweeted about his arrest before the sites were restricted.
Another MP from the party who is currently abroad, Ertugrul Kurkcu, told the BBC that the detentions were “totally unlawful”.
He said: “This crackdown tonight is nothing to do with procedural law, criminal law, any law whatsoever or the constitution. This is an unlawful hijacking of HDP parliamentarians.
“The Turkish government is heading towards a dictatorship of Nazi style [sic]. Will the Turkish government abide by the internationally accepted standards of parliamentary democracy? This is the basic question.”
Last month, the co-mayors of Turkey’s largest Kurdish-majority city were detained, also as part of a terrorism investigation.
About 100,000 public sector employees with alleged links to the coup’s alleged mastermind were subsequently purged from their jobs.
The HDP entered the Turkish parliament for the first time last year, when it won 59 seats and became the country’s third-largest party.
Mr Demirtas has accused the ruling party of orchestrating nationalist attacks.
Turkish politicians normally have immunity from prosecution but this was removed from the HDP earlier this year.
Turkey HDP: Blast after pro-Kurdish leaders Demirtas and Yuksekdag detained