ANKARA — The European Union on Thursday criticized Turkish authorities for stripping a prominent pro-Kurdish legislator of his parliamentary seat and seeking to shut down his political party, saying these moves add to concerns over the "backsliding of rights" in Turkey.

Omer Faruk Gergerlioglu, a human rights advocate and lawmaker from the People's Democratic Party, or HDP, was expelled from parliament on Wednesday after an appeals court upheld his conviction on terrorist propaganda charges over a social media posting. Gergerlioglu says the case against him was politically motivated, and argues that he was unjustly stripped of his seat in parliamentary before Turkey's highest court reviews his case.

Hours later, a top prosecutor filed a lawsuit with Turkey's Constitutional Court seeking to disband the HDP for alleged ties to the banned Kurdistan Workers' Party, or PKK, and to bar more than 600 of its members from politics for five years. 

The moves in parliament and by the prosecutor were the latest in a years-long crackdown on the second-largest opposition party in parliament. Dozens of elected HDP lawmakers and mayors — including former co-chair Selahattin Demirtas — as well as thousands of party members have been arrested on terror-related accusations. 

"Closing the second largest opposition party would violate the rights of millions of voters in Turkey," EU foreign affairs chief Josep Borrell and Oliver Varhelyi, the EU enlargement commissioner said in a joint statement. "It adds to the EU's concerns regarding the backsliding in fundamental rights in Turkey and undermines the credibility of the Turkish authorities' stated commitment to reforms."

The United States also spoke out against the steps taken against Gergerlioglu and his party, with State Department spokesman Ned Price saying efforts to dissolve the HDP "would unduly subvert the will of Turkish voters, further undermine democracy in Turkey, and deny millions of Turkish citizens their chosen representation."

A senior Turkish official however, called for respect for Turkey's judiciary and insisted the HDP has "organic ties to the PKK."

"HDP's senior leader and spokespeople, through their words and deeds, have repeated and consistently proved that they are the PKK's political wing," said Fahrettin Altun, the presidential communications director, on Twitter. 

The PKK is considered a terror organization by Turkey, the EU and the U.S.