Turkish opposition leader Özgür Özel has vowed to fight a court ruling removing him and fellow party leaders, in the latest legal move that helps cement President Recep Tayyip Erdoğan's grip on power.

The appeal court declared the 2023 leadership election in Özel's Republican People's Party (CHP) null and void, prompting thousands of demonstrators to gather outside party headquarters in Ankara.

"We are experiencing a dark day for Turkish democracy," said Özel, who vowed to challenge the ruling in the courts and with Turkey's supreme election council (YSK).

Justice Minister Akin Gürlek insisted the ruling "reinforces our citizens' trust in democracy".

Until Gürlek was appointed justice minister by President Erdoğan earlier this year, he was the chief prosecutor in Istanbul who spearheaded investigations targeting the opposition, and the city's highly popular mayor Ekrem Imamoğlu in particular.

Imamoğlu, who is still considered Erdoğan's biggest political rival, has been in jail for over a year and it was Gürlek who accused him of corruption offences which command more than 2,000 years behind bars.

The supreme election council was meeting on Friday to hear the opposition party's objection to the court's decision.

In its ruling on Thursday, the appeals court in Ankara overturned a 2025 ruling by a lower court that threw out allegations of vote buying during the CHP primary which made Özgür Özel opposition leader.

The appeals court decided that Özel should be replaced by Kemal Kilicdaroğlu, a 77-year-old party veteran who was defeated by Erdoğan in the 2023 presidential election and then voted out as party leader.

The ruling also means the party's entire executive is replaced and it is thought the decisions it has made are no longer recognised.

BBC Turkish correspondent Ayşe Sayin said the ruling came as a shock at CHP headquarters, with some members moved to tears. She described tense moments when some party members removed a photo of Kilicdaroğlu from the wall, while others chanted the former leader was a "traitor".

Turkey's stock market plummeted 6% late on Thursday before recovering some of that ground on Friday morning.

Erdoğan has led Turkey since 2003, as prime minister and then as president, and Özel has accused Erdoğan's AK party of pursuing a strategy to "eliminate its rivals". The 72-year-old leader can only run for president again if he calls early elections before 2028 or changes the constitution.

The CHP leader bitterly condemned Thursday's court ruling as a "judicial coup" and the party's executive rejected the court's authority.

"These coup plotters don't come with tanks, cannons, rifles or camouflage; these coup plotters come in judges' and prosecutors' robes," Özel complained.

He also rejected speculation that the party would split and he would head a breakaway party: "Tenants leave, homeowners are to stay."

However, any legal action that challenges the ruling in Turkey's Court of Cassation is expected to drag on. There are suggestions that Özel and his supporters may have to move towards forming another party with a view to fighting the next election.

There have also been defiant messages from two of the CHP's most popular figures.

Istanbul's jailed and suspended mayor, Ekrem Imamoğlu, posted a message on social media calling on Turks to "stand together for their country".

Ankara Mayor Mansur Yavaş said the aim of the court ruling was to divide Turkey's biggest opposition party and render it ineffective. He warned that with the economy flagging, there was a strong likelihood that Turkey's political leaders could move towards snap elections in the coming period.

In a separate development on Friday, President Erdoğan ordered the closure of Istanbul's independent Bilgi University, which has some 22,000 students from home and abroad.

Law professor Yaman Akdeniz said a university built up over 30 years had in effect been shut down overnight: "An institution where I've taught for 15 years, one we've nurtured alongside thousands of young people, is being disregarded."

Turkey's Council of Higher Education said it was taking steps to ensure current students would not suffer hardship.