The two co-leaders of Turkey's pro-Kurdish opposition party have been detained along with other MPs, BBC news reports.

Selahattin Demirtas and Figen Yuksekdag were detained at their respective homes "as part of a counter-terrorism inquiry," security sources quoted by Anadolu news agency said.

At least nine other MPs from the People's Democracy Party (HDP) were also taken into custody.

Twitter, Facebook, YouTube and Whatsapp were reported to be inaccessible inside Turkey shortly after the detentions, even when users tried to circumvent restrictions using a virtual private network (VPN).

Demirtas had tweeted about his arrest before the sites were restricted.

Police searched the HDP's head offices in central Ankara as well as making the arrests.

Turkey claims that the HDP has links to the Kurdistan Workers' Party (PKK), a militant group, but the party strongly denies this.

The PKK is deemed a terrorist organisation by the U.S., the European Union and Turkey.

Turkey remains under a state of emergency that was imposed after a failed coup in July. The emergency allows President Recep Tayyip Erdogan and his cabinet to bypass parliament when drafting new laws and to restrict or suspend rights and freedoms.