The pro-Kurdish leftist Peoples' Democratic Party (HDP) is Turkey's second-biggest opposition party but in the past few years has been the subject of multiple prosecutions.

Its two former leaders have languished in jail since they were arrested and prosecuted in 2016 after being stripped of their immunity while members of parliament. This occurred when President Recept Tayyip Erdogan unleashed a sweeping political crackdown after surviving a coup attempt.

Turkish prosecutors sent 33 legal proceedings to the parliament on Tuesday in a bid to remove the immunity of at least nine HDP MPs, speaker Mustafa Sentop was quoted as saying by the Anadolu state news agency.

He added some of the notices concerned HDP's co-leader Pervin Buldan, who is accused of involvement in 2014 pro-Kurdish protests that left 37 dead.

The unrest erupted after Turkish troops took no action while Islamic State jihadists fought to capture the mainly Kurdish northern Syrian border town of Kobane.

Buldan was among nine MPs named by the Ankara chief public prosecutor on Friday in a case of 108 suspects linked to the 2014 unrest.

The government has already jailed dozens of mayors and other HDP officials, and replaced the majority of HDP municipalities won in 2019 with trustees.

The HDP stands accused of links to the outlawed Kurdistan Workers' Party (PKK) but the party strongly denies formal ties.

The PKK has waged an insurgency against the Turkish state since 1984 and is blacklisted as a terrorist organisation by Ankara and its Western allies.

But the HDP has come under further scrutiny in the past few weeks, especially after the leader of the ruling party's ultranationalist partner, Devlet Bahceli, repeatedly called for the HDP to be shut down.

The political pressure redoubled after Erdogan accused the PKK of executing 12 Turks and an Iraqi who were being held hostage in northern Iraq on February 14.

The PKK says the 13 were killed by Turkish bombs.

"PKK and HDP are one and the same," Erdogan's communications director Fahrettin Altun tweeted that same day.

The court of cassation last week also upheld a two-year prison sentence for HDP MP Omer Faruk Gergerlioglu for "making terrorist propaganda" in a social media post.