Tens of thousands of people protested in Ankara ahead of a key court hearing today targeting Turkey's main opposition Republican People's Party (CHP), BGNES reports.
It could lead to a change in its leadership.
Critics say the vote-buying case is a politically motivated attempt to undermine Turkey's oldest political party, which won a landslide victory over President Recep Tayyip Erdogan's Justice and Development Party (AKP) in local elections in 2024 and is enjoying a surge in popularity in opinion polls.
The CHP denies the charges and accuses the government of trying to disarmed it as an opposition force.
Her popularity soared after she led the biggest street protests in Turkey in a decade in March, sparked by the arrest of her presidential candidate, Istanbul Mayor Ekrem Imamoglu.
At least 50,000 people took part in a rally in the capital Ankara, where HDP leader Özgür Özel urged the crowd to take a "stand firm" against the legal attack.
"This case is political, the accusations are slander. This is a coup and we will resist," he said.
"We are facing the grave consequences of the Turkish government abandoning the "democracy train" and chooses to rule through oppression, not through the ballot box," added Özel.
The case seeks to overturn the results of the November 2023 PDP congress on the grounds of vote rigging. The congress removed longtime party chairman Kemal Kilicdaroglu and elected Özel.
The indictment names Kilicdaroglu as the injured party and seeks sentences of up to three years in prison for İmamoğlu and ten other mayors and HDP officials for "vote fraud".
If the case succeeds, Özel could be removed from his leadership post.
He and eight other lawmakers are also being investigated on similar charges, but their cases are being heard by a parliamentary committee because they all have immunity.
On September 2, a court removed the leadership of the HDP's Istanbul branch over allegations of vote-buying at the provincial congress and appointed a governor to take over, a move seen as a test of the case today.
The decision sparked angry protests and sent the stock market down 5.5%, raising concerns that the outcome could harm Turkey's fragile economy.
If the Ankara court annuls the results of the AKP congress, it could herald the return of former leader Kilicdaroglu, who suffered a series of electoral defeats that left the party in crisis.
Experts say such a decision is likely to spark serious internal conflict at the AKP headquarters in Ankara.
In an attempt to defend its leadership, the AKP has called an extraordinary congress on September 21. This means that if the court removes Özel and reinstates Kilicdaroğlu, members can simply vote for Özel six days later.
The detained mayor of Istanbul condemned the hearing as an attack on democracy.
"This is not a question of the HDP, but of the existence or lack of democracy in Turkey," Imamoğlu said after appearing in court on unrelated charges.
When Özel took office in November 2023, the HDP was in crisis, but by March 2024 he had led the party to a landslide victory in local elections.
Özel became the face of the large-scale street protests that erupted after İmamoğlu was arrested, and has since led weekly protests across Turkey that draw huge crowds, angering Erdogan.