The move left the way clear for the candidate from President Recep Tayyip Erdogan's ruling Justice and Development (AKP) party to take office in the city of Van with just 27 percent of the vote, the DEM party added in a statement.

During nationwide municipal elections Sunday, DEM's Abdullah Zeydan had garnered over 55 percent of the vote in Van, which lies on Lake Van around 80 kilometres (50 miles) from Turkey's eastern border with Iran.

DEM said that just two days before the vote, the justice ministry had reversed a court decision that restored Zeydan's right to stand for election.

He had been elected to parliament on the HDP (now DEM) ticket in 2015 and arrested in 2016 with a dozen other deputies after criticising the Turkish army's aerial bombardments of outlawed Kurdish militants in the southeast.

"We reject the decision made by the provincial election board in Van to give mandate to the AKP candidate even though our party won by a large margin," DEM said in a statement.

Zeydan could still appeal the decision that sparked protests in Van province, home to around 1.1 million people.

Television footage showed hundreds of protesters gathered outside DEM's Van headquarters in a show of solidarity.

"Abdullah Zeydan is our honour," they chanted. "Government-appointed trustees cannot deter us."

Police fired tear gas and water cannon to break up the protest.

DEM urged Erdogan's government, which was dealt a harsh nationwide blow in the elections, to "respect the will of the people" in Van.

Ozgur Ozel, leader of the opposition CHP Party whose candidates won Istanbul and Ankara as well as inner Anatolian cities in Sunday's vote, backed DEM, calling the overturning of Zeydan's victory a "disgrace".

DEM -- accused by authorities of links to outlawed Kurdish militants -- on Sunday swept large towns in the Kurdish-majority southeast, including the region's largest city Diyarbakir.

Following 2019 elections, fifty-two mayors elected in the southeast on the HDP (now DEM) ticket were stripped of office and replaced by state-appointed administrators for alleged ties to Kurdish militants.

That followed a 2016 coup attempt against Erdogan's government, which prompted a massive crackdown on opponents of all stripes -- even though the movement led by US-exiled cleric Fethullah Gulen was the main target.