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 Recep Tayyip Erdogan's main rival, already jailed pending corruption charges he denies.

Imamoglu, at the centre of a sprawling legal crackdown on the main opposition party, has been jailed since March 23 pending trial. The 54-year-old 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.

The March arrest came with the opposition Republican People's Party (CHP) poised within days to name the two-term mayor its official presidential challenger to Erdogan, who has run Turkey for more than two decades after becoming prime minister in 2004.

Crackdown on the opposition

The next election is set for 2028, but Erdogan has reached his two-term limit as president, after amending the constitution in 2018.

The 72-year-old president is again seeking to amend the constitution, but has denied it is a move intended to keep him in power.

Erdogan faced his worst electoral defeat last year when Imamoglu's CHP swept Turkey's major cities and defeated his ruling AK Party in former strongholds in nationwide municipal elections.

Born in 1971 in the Black Sea province of Trabzon, Imamoglu studied business administration at Istanbul University before entering his family's construction business. 

Imamoglu joined the CHP in 2008 and became mayor of Istanbul's Beylikduzu district in 2014.

He won the citywide election in 2019. A court annulled his initial victory, only for him to win the rerun election by an even wider margin.

He won re-election as Istanbul's mayor in March 2024. That happened despite an earlier legal battle, when he was sentenced in 2022 to two-and-a-half years in prison for insulting public officials, though an appeals court has yet to rule in the case.

Another case last year accused him of tender-rigging. His supporters view these charges as politically motivated attempts to sideline him, a claim Erdogan and the AKP deny.

Still another investigation accuses Imamoglu and six others of aiding the outlawed Kurdistan Workers' Party (PKK), considered a terrorist organisation by Turkey and its Western allies.

Imamoglu's troubles are part of a months-long crackdown on the opposition. Early last month, three CHP mayors of Istanbul districts were suspended by federal authorities.

While initially focused on Istanbul, this week Turkish authorities detained 109 people, including opposition party members and a former mayor, in Izimir, the Anadolu state news agency said.