Turkish authorities detained an Istanbul district mayor and about 40 other officials Friday in what appears to be an escalation of the government’s crackdown on the country’s opposition.

İnan Güney, the mayor of Istanbul’s Beyoğlu district, was taken into custody as part of an investigation into alleged corruption, the state-run Anadolu Agency reported. Several of his close aides, including his bodyguard and some of his advisers, were also detained, according to local media reports.

Güney is a member of Turkey’s main opposition party, the secular Republican People’s Party (CHP). His arrest comes five months after the CHP’s Ekrem İmamoğlu, the popular opposition mayor of Istanbul, was jailed over corruption allegations.

İmamoğlu is the main political rival of Turkish President Recep Tayyip Erdoğan and has been nominated as the CHP’s candidate for the country’s 2028 presidential election. He denies wrongdoing and says his jailing is politically motivated.

His opposition party has steadily risen in popularity, performing well in regional elections last year, winning a fiercely fought mayoral election in Istanbul in part by turning districts traditionally held by Erdoğan’s ruling Islamist party. In those elections, the CHP also won Turkey’s capital, Ankara, as well as Izmir, Bursa and Adana, pushing its support to 37. 8 percent nationwide and capturing 35 of 81 provinces. 

While Erdoğan’s government insists the arrests aren’t politically motivated, rights groups say they’re meant to stop the CHP’s growing influence.

İmamoğlu’s arrest in March sparked massive protests, which Ankara suppressed with thousands of arrests. Since then, Turkish authorities have arrested dozens more opposition figures.