Turkish prosecutors charged Istanbul’s mayor Ekrem Imamoglu on Friday with falsifying his university diploma, a new case threatening more years in prison for President Tayyip Erdogan’s main rival, already jailed pending corruption charges he denies.

Imamoglu, at the center of a sprawling legal crackdown on the main opposition party, has been jailed since March 23 pending trial. He denies the allegations against him, which his party says are orchestrated to protect Erdogan in power.

His indictment over his diploma was reported by Milliyet newspaper, which said prosecutors were seeking eight years and nine months of prison time for the new charges. Reuters could not immediately obtain the document.

On March 18, Istanbul University said it had annulled Imamoglu’s diploma. He was detained a day later on the corruption charges, triggering Turkey’s largest protests in a decade, and later jailed pending trial.

His detention has drawn sharp criticism from opposition parties and some foreign leaders, who call the case politically motivated and anti-democratic. The government denies the case is political.

Imamoglu is the main opposition Republican People’s Party’s presidential candidate in any future election. He won re-election as mayor in March last year by a wide margin against a candidate from Erdogan’s ruling AK Party.