The former co-leader of the pro-Kurdish People's Democratic Party (HDP) has been sentenced to 3-1/2 years in jail for insulting the president, his lawyer said.
Selahattin Demirtas is already in jail awaiting a final verdict, but could face life imprisonment based on separate charges related to protests in 2014.
The sentence against Mr Demirtas came less than a week after a top prosecutor filed a lawsuit demanding a ban on the HDP.
The move followed months of intensified calls from President Tayyip Erdogan's nationalist allies for the HDP to be banned for alleged ties to the outlawed Kurdistan Workers Party (PKK).
The HDP denies the charges and has called the move a "political coup."
Ramazan Demir, a lawyer for Mr Demirtas, said on Twitter that Monday's sentence was one of the longest given on the charge of insulting the president, which is a crime in Turkey.
He said the court disregarded a European Court of Human Rights ruling, which called for Mr Demirtas' immediate release saying his detention was a cover for limiting pluralism.
Mr Demirtas had criticised Mr Erdogan in a speech in December 2015, saying the president had "fluttered from corridor to corridor" during a conference in Paris, hoping to have a picture taken with Russian President Vladimir Putin.
Those comments came after Turkish forces shot down a Russian warplane over Syria, when ties between the countries were at their worst since the end of the Cold War.
"My only regret over the speech is that I said too little," Mr Demirtas told the court, according his lawyer.
Mr Demirtas, one of Turkey's most prominent politicians who has been in jail for 4-1/2 years, could face a life sentence if convicted in the main case against him related to 2014 protests in the mainly Kurdish southeast.
Last week, Turkey's parliament also stripped prominent HDP deputy and human rights advocate Omer Faruk Gergerlioglu of his seat after he was found guilty of spreading terrorist propaganda for sharing a news link on Twitter.
The PKK is designated a terrorist group by Turkey, the United States and European Union. It has fought an insurgency against the state in mainly Kurdish southeast Turkey since 1984. More than 40,000 people have been killed in the conflict.