Police have fired tear gas to disperse crowds of demonstrators in Istanbul.
Many are angry about the arrests overnight of several pro-Kurdish politicians.
Fighting broke out between police and protesters in a number of cities.
Officials say 10 protesters were detained in Ankara, five in Antalya and seven in Istanbul.
What sparked the protests?
The Turkish authorities have arrested the leaders of the country’s main pro-Kurdish opposition party as part of an ongoing terrorism inquiry.
The arrests have drawn criticism from the UN and EU, as part of a widening crackdown on dissent under President Recep Tayyip Erdogan.
Who has been arrested?
Officials say Selahattin Demirtas and Figen Yuksekdag, co-leaders of the People’s Democratic Party (HDP) are in prison, having been held in overnight raids.
Ten other HDP lawmakers were also detained, although some were later released.
“I will not hesitate to be held accountable in front of a fair and impartial judiciary. There is nothing I cannot answer for,” Demirtas said in a statement to the prosecutor.
But I refuse to be an actor in this judicial theatre just because it was ordered by Erdogan, whose own political past is suspicious,” he continued.
Police also raided and searched the party’s head office in central Ankara.
Police cars and armed vehicles had closed off the entrance to the street.
What has the UN said?
The arrest of elected members of parliament’s third-largest party, along with the detention or suspension of more than 110,000 officials since a failed coup in July, may “go beyond what is permissible”, according to senior human rights official Ravina Shamdasani.
Has there been other international reaction?
Germany and Denmark have summoned Turkish diplomats over the Kurdish detentions.
The European Parliament President Martin Schulz says the actions “call into question the basis for the sustainable relationship between the EU and Turkey”.
EU foreign policy chief Federica Mogherini says she is “extremely worried” by the arrests and has called a meeting of EU national envoys in Ankara.
There is 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.
What has Turkey said?
“Turkey is a nation of laws, nobody has preferential treatment before the law. What has been done is within the rule of law,” Prime Minister Binali Yildirim told reporters.
“Politics cannot be a shield for committing crimes.”
Turkish Foreign Minister Mevlut Cavosoglu has accused EU member states of supporting the PKK.
He has dismissed the bloc’s criticism as “unacceptable”.
What has the HDP said?
The HDP made history last year by becoming the first Kurdish party to win 10 percent of the vote and enter parliament.
The party says the detentions risk triggering a civil war.
Has there been a response from the PKK?
In a video message posted on a website close to the PKK, one of the militant group’s top commanders, Murat Karaliyan, said the group would intensify its three-decade old armed struggle against Turkey.
He called on Kurds, the country’s largest minority, to react.
Hours after the detentions, a car bomb planted by suspected Kurdish PKK militants killed nine people and wounded more than 100 near a police station in the southeastern city of Diyarkbakir.
Some of those arrested were being held in the city.
Was access blocked to social media?
Access was “throttled” to social media, including Twitter, WhatsApp, Youtube and Facebook.
It made the sites so slow they were effectively impossible to use.
A ban was imposed on media coverage of the car bomb.
Asked about this, Binali Yildirim said access would return to normal “once the danger is removed”.
Southeastern Turkey has been rocked by political turmoil and violence for more than a year after after the collapse of a ceasefire with the PKK.
The group is considered a terrorist organisation by the US and EU as well as Turkey.
Erdogan and the governing 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.