Selahattin Demirtas and Figen Yuksekdag, co-leaders of the Peoples' Democratic Party (HDP), were jailed pending trial after being held in overnight raids, officials said. Ten other HDP lawmakers were also detained, although some were later released.
The arrest of elected members of parliament's third largest party, and the detention or suspension of more than 110,000 officials since a failed coup in July, may "go beyond what is permissible", the UN human rights office said.
The United States expressed deep concern, while Germany and Denmark summoned Turkish diplomats over the Kurdish detentions, and European Parliament President Martin Schulz said the actions "call into question the basis for the sustainable relationship between the EU and Turkey".
Turkish Prime Minister Binali Yildirim told reporters: "Turkey is a nation of laws, nobody has preferential treatment before the law ... What's been done is within the rule of law."
File image of co-leaders of the pro-Kurdish and pro-minority political party Peoples' Democratic Party (HDP) Figen Yuksekdag (R) and Selahattin Demirtas (L) Source: AAP
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The HDP lawmakers were arrested after they refused to give testimony in a probe linked to "terrorist propaganda". "Politics can't be a shield for committing crimes," Yildirim said.
Hours after the detentions, a car bomb planted by suspected militants killed nine people and wounded more than 100 near a police station in the southeastern city of Diyarbakir where some of the lawmakers were being held.
A news outlet linked to the Islamic State jihadist group said Friday that its fighters were behind a bombing that killed nine people in Kurdish-dominated southeastern Turkey, according to US-based monitors.
"An insider source for Amaq Agency: Fighters from the Islamic State detonated an explosives-laden vehicle parked in front of a Turkish police headquarters in Diyarbakir in southeastern Turkey," the SITE Intelligence Group cited the IS-affiliated Amaq news agency as saying.
The HDP, which made history last year by becoming the first Kurdish party to win 10 percent of the vote and enter parliament, said the detentions risked triggering civil war.
In a video message on a website close to the PKK, one of the militant group's top commanders, Murat Karayilan, said the group would intensify its three-decade old armed struggle against Turkey and called on Kurds, the country's largest minority, to react.
Social media blocked
Access was restricted to social media including Twitter, WhatsApp, YouTube and Facebook - making them so slow they were effectively impossible to use - and a ban imposed on media coverage of the car bomb. Asked about the measures, Yildirim said access would return to normal "once the danger is removed".
U.S. State Department spokesman John Kirby said: "The United States is deeply concerned by the Turkish government's detentions of opposition members of parliament ... and by government restrictions on Internet access today."
He also condemned Friday’s bombing and urged the PKK to "cease its senseless brutal attacks".
The arrests of elected lawmakers from the HDP, which won more than five million votes at the last general election, heightened concern among Western allies about the political direction of Turkey, a NATO member and a buffer between Europe and the conflicts in Syria and Iraq.
European Union foreign policy chief Federica Mogherini said she was "extremely worried" by the arrests, and raised her concerns in a telephone call with Turkey's foreign and EU affairs ministers late on Friday.
People with banners reading 'SOS Turkey! Democracy, human rights, freedom... are trodden by an arbitrary government' demonstrate against the arrests (AAP) Source: AAP
German Foreign Minister Frank-Walter Steinmeier said Ankara had a right to fight terrorism but could not use it to justify gagging opponents.
Turkish Foreign Minister Mevlut Cavusoglu accused EU member states of supporting the PKK and dismissed the bloc's criticism as "unacceptable". Turkey began negotiations to join the EU in 2005, but has made glacial progress, and there is no prospect of it joining anytime soon.
Currency falls
Southeastern Turkey has been rocked by political turmoil and violence for more than a year after the collapse of a ceasefire with the PKK, deemed a terrorist organisation by the United States and European Union as well as Turkey.
Erdogan and the ruling AK Party accuse the HDP of links to the PKK. The HDP denies direct links and says it is working for a peaceful resolution of the Kurdish conflict.
The lira hit a new low against the dollar of 3.14 after the arrests, while the cost of insuring Turkish government debt against default hit its highest in over a month.
In a statement on Twitter, still accessible in Turkey through virtual private networks (VPNs), the HDP called for the international community "to react against the Erdogan regime's coup", while party spokesman Ayhan Bilgen described the detentions as an attempt to provoke a civil war.
"I will not hesitate to be held accountable in front of a fair and impartial judiciary. There is nothing I cannot answer for," the arrested Demirtas said in a statement to the prosecutor, which was shared by HDP lawmaker Besime Konca.
"But I refuse to be an actor in this judicial theater just because it was ordered by Erdogan, whose own political past is suspicious," he said.
Police also raided and searched the party's head office in central Ankara. Police cars and armed vehicles had closed the entrances to the street of the HDP headquarters.
A group of protesters chanting slogans tried to reach the party offices, but were stopped by police before they could enter the street, a Reuters witness said.
The HDP is the third-largest party in the 550-seat Turkish parliament, with 59 seats. Parliamentarians in Turkey normally enjoy immunity from prosecution, but the immunity of many lawmakers, including HDP deputies, was lifted earlier this year.
