A car bomb killed eight people and wounded more than 100 in southeastern Turkey's largest city on Friday, Prime Minister Binali Yildirim said, hours after police detained the leaders of the mostly Kurdish region's biggest political party.

The blast struck near a police station in Diyarbakir where some of the party leaders were being held in a terrorism probe. It tore the facades off buildings and firefighters were searching for people trapped by debris.

A spokesman for the pro-Kurdish Peoples' Democratic Party (HDP), parliament's second-biggest opposition grouping, said the detention of its two leaders and at least nine other lawmakers risked triggering civil war.

Yildirim told reporters that elected officials who incite and encourage terrorism must face legal proceedings and that they were detained because they had refused to give testimony.

The arrests will heighten concern among Western allies about a deepening crackdown on dissent under President Tayyip Erdogan and about political stability in Turkey, a NATO member and a buffer between Europe and the conflicts in Syria and Iraq.

The arrests, which drew swift condemnation from the European Union, come as Turkey has detained or suspended more than 110,000 officials in the wake of a failed July coup. Turkey is considering reintroducing the death penalty, and earlier this week journalists from a leading opposition newspaper were detained.

"Very bad news from Turkey. Again. Now HDP members of parliament are being detained," the European Parliament's Turkey rapporteur, Kati Piri, said on Twitter of a country that is seeking membership of the EU.

EU foreign policy chief Federica Mogherini said she was "extremely worried" by the arrests and had called a meeting of EU ambassadors in Ankara.

Southeastern Turkey has been rocked by political turmoil and violence for more than a year after the collapse of a ceasefire with the Kurdistan Workers Party (PKK) militant group, which has waged a three-decade insurgency for Kurdish autonomy.

Access was blocked to social media, including Twitter, Whatsapp, YouTube and Facebook, an internet monitoring group said, and a reporting ban was imposed on coverage of the blast. Asked about the measures, Yildirim said the situation would go back to normal "once the danger is removed".

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.

"The reports of increased crackdown and talk of the reintroduction of the death penalty raises concerns about the future trajectory of FDI flows and even EU accession," said Manik Narain, an emerging markets strategist at UBS in London.

Erdogan and the ruling AK Party accuse the HDP of links to the PKK, which is deemed a terrorist organisation by the United States and the European Union. The HDP, which won more than 5 million votes in the last general election a year ago, denies direct links.

The government introduced a nationwide state of emergency after a failed military coup on July 15 which gave it broad powers to round up suspects linked to the putsch. More than 110,000 civil servants, soldiers, police, judges and other officials have been suspended or detained, as have journalists.

The authorities have also used the emergency powers to round up pro-Kurdish opposition activists and politicians, including Diyarbakir's joint mayors, who were detained late last month, and has closed all major Kurdish media outlets.

"JUDICIAL THEATER"Police raided the Ankara house of Figen Yuksekdag, HDP co-chairwoman, and Selahattin Demirtas, the party's other leader, in Diyarbakir. A court official said the prosecutor was seeking the formal arrest of Demirtas and that both he and Yuksekdag were in court after police questioning.

"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, 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.

In a statement on Twitter, still accessible in Turkey through virtual private networks (VPN), 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.

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.