Turkish authorities arrested the co-leaders of the pro-Kurdish opposition Peoples' Democratic Party (HDP) on Friday on terrorism-related charges as the post-coup crackdown in the country continues.
Ten other HDP politicians were also detained, although some were later released, while access to social media and some websites were blocked after the detentions, according to an internet monitoring group.
Friday's arrests came as nine people were killed and nearly 100 others wounded in a car bomb attack in Turkey's southeast.
Turkey Blocks - a monitoring group that provides a live feed of internet shutdown incidents in the country - said Facebook, Twitter and YouTube were all "blocked by throttling", a method of slowing certain websites to the point where they are unusable.
Protests erupted around the country following the arrests and some demonstrators were detained by riot police.
The move to arrest Figen Yuksekdag, HDP co-chairwoman, and Selahattin Demirtas, the party's other leader, came after their detention in overnight raids early on Friday.
The HDP is the third largest party in the 550-seat Turkish parliament with 59 seats, and the main political representative of the Kurdish minority.
Internet issues "temporary"
The arrests drew condemnation from European leaders and rights groups, as well as Turkey's main opposition Republican People's Party (CHP).
EU foreign policy chief Federica Mogherini said on Twitter she was "extremely worried" over the detentions and would call a meeting of EU ambassadors in Ankara.
German Foreign Minister Frank-Walter Steinmeier summoned Turkey's envoy to the foreign ministry in Berlin, a ministry source said, adding Germany "could not remain silent".
But politicians should "pay the price" if they engage in "terror", Prime Minister Binali Yildirim said in reaction to the criticism.
"Politics cannot be a shield for committing a crime. Turkey is a state of law," Yildirim told journalists.
Yildirim also said internet connection issues were part of measures taken for "security purposes" and they were "temporary".
"These precautions can be taken for security purposes. These are temporary precautions. Once the threat is eliminated, everything goes back to normal," he said.
The turbulence also battered the Turkish lira, which fell to a new historic low against the dollar.
HDP co-leader Selahattin Demirtas was arrested at his home in the southeastern city of Diyarbakir, the largest city in Turkey's mainly Kurdish southeast, while Figen Yuksekdag was held in the capital Ankara, Anadolu said.
Police also raided the party's head office in central Ankara. Television images showed party officials arguing with police during the raid.
Hundreds of arrests have been made in recent months since the government acquired state-of-emergency powers after the failed coup on July 15.
Authorities say they have been going after anyone suspected of links to Fethullah Gulen, a US-based religious leader accused of orchestrating the coup attempt, as well as the outlawed Kurdistan Workers' Party (PKK).
Critics, though, say the government is using the coup as a pretext to muzzle all dissent.
The Turkish government accuses HDP of having links to the PKK, which is deemed a terrorist organisation by the United States, the European Union, and Turkey. HDP denies any ties.
On Friday, a top PKK commander said the armed group will intensify its struggle.
Murat Karayilan said in a video message published on a website close to the PKK it was "very important" for Kurdish people to react against the detentions of the HDP politicians.
Earlier Friday, two police officers and seven civilians were killed in a blast claimed by the PKK in Diyarbakir, where Demirtas was arrested.
Demirtas and Yuksekdag had been targeted by several investigations in recent months, but this was the first time either was arrested. Their arrests follow a resolution by parliament earlier in the year allowing the immunity of MPs to be lifted.
"HDP call international community to react against Erdogan Regime's coup," the party said on Twitter, referring to President Recep Tayyip Erdogan.
Earlier this week Gultan Kisanak, the HDP mayor of Diyarbakir, and co-mayor Firat Anli were arrested over alleged membership of the PKK.
In September, the government similarly removed 28 mayors and other administrators, mostly from the HDP, and appointed trustees in their place.
More than 110,000 civil servants, soldiers, police, judges and other officials have been suspended or detained since the July coup attempt.
Scores of opposition media organisations have been shut down, including pro-Kurdish ones such as IMC TV, the Dicle news agency, and the Ozgur Gundem newspaper.