Small demonstrations also took place in Wales in Great Britain and Netherlands. European states and the US expressed concerns over the detentions but condemned the fatal bombing in the city of Diyarbakir.
Protesters on Friday clashed with police in various cities of Turkey over the arrest of senior Peoples' Democratic Party (HDP) parliamentarians, including the two chairpersons, in an ongoing terror investigation.
Co-chairs Figen Yuksekdag and Selahattin Demirtas and parliamentary leader Idris Baluken were among the 12 lawmakers arrested by the police for failing to answer summons linked to a terrorism probe.
Turkish police detained several demonstrators from outside the Diyarbakir courthouse on Friday where HDP leaders were being questioned.
Angry protesters blocked the roads in Istanbul and Ankara, refusing to disperse. Police had to resort to tear gas after several warnings went unheeded. Several people were detained during the clashes.
Marches and protests have been prohibited by the governor for security reasons in Ankara till the end of November.
HDP members are not the only ones summoned by the court recently. The chairpersons of Nationalist Movement Party (MHP) and Republican People's Party (CHP) were also called upon by the court. Both testified in cases against them.
Meanwhile, the arrests of HDP lawmakers and the fatal bomb blast in the city of Diyarbakir drew varied reactions from other countries.
German Foreign Minister Frank-Walter Steinmeier said Ankara had a right to fight terrorism, but without "gagging" opposition. Germany and Denmark also summoned Turkish diplomats to discuss the issue.
"The EU is very disquieted by the ongoing violence in the south-east over the last 12 months, which cannot continue," a written statement released by the EU foreign policy chief Federica Mogherini's office said.
The bloc expressed condolences to the families of the victims killed and wished a speedy recovery to the injured, adding that they considered the PKK as a terrorist organisation.
"Significant action has been taken against the PKK in the European Union and its member states in the recent years and months," according to the statement.
While US State Department spokesperson John Kirkby condemned Friday's bombing and PKK, he also expressed concern over the detention of opposition party members and patchy internet access.
European Parliament President Martin Schulz questioned how Friday's actions would impact the relationship between EU and Turkey.