Footage from the scene showed those inside shouting and throwing objects towards the entrance, and police being sprayed with hoses.
Party leader Özgür Özel had vowed to defy an appeal court decision on Thursday which declared his election null and void, in a move further cementing President Recep Tayyip Erdoğan's grip on power.
The 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.
Turkish media reported that Kilicdaroğlu's representatives had requested the police carry out "necessary procedures" to hand over the headquarters to the new leadership, claiming they had so far been barred entry.
The city's governor then instructed the police to "implement the court decision", according to a statement from his office.
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"We are under attack," Özel said in a video message shared on X as the security forces sought entry to the building, where clashes were also reported between his supporters and those of Kilicdaroğlu.
Özel later emerged from the premises, telling crowds gathered outside: "They tried to uproot and throw us out - to where?"
He said the party would "from now be on the streets or in the squares, marching towards power".
He then set off towards the Turkish parliament, leading hundreds of supporters through the streets of Ankara.
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Ousted CHP leader Özgür Özel waved to supporters atop a police vehicle as they marched through the Turkish capital
The court's decision overturned a 2025 ruling by a lower court that threw out allegations of vote buying in the party election which made Özel leader. (Source: BBC)
