The former mayor of Istanbul is due to become the central defendant in a mass corruption trial that his opposition party and rights groups say is politically motivated.

Ekrem Imamoglu, 55, faces more than 140 charges including corruption and running a criminal organisation. Prosecutors are seeking a jail term of up to 2,430 years.

He and his Republican People's Party (CHP) deny any wrongdoing. They accuse Turkish President Recep Tayyip Erdogan and his allies of launching a crackdown over the leader's decline in popularity.

Prior to the trial, which begins on Monday, Imamoglu was widely seen as one of the few politicians capable of defeating Erdogan, who has been at the top of Turkish politics since 2003.

Imamoglu struck a defiant tone ahead of the trial, in comments given to news agency Reuters by his legal team last month, urging Erdogan to call elections immediately.

Another 407 of Imamoglu's supporters have also been put on trial, accused of running a criminal corruption network headed by the opposition figure, who prosecutors describe as its "founder and leader".

The former mayor was arrested in March last year on the day he was named as the leader of the CHP and its presidential candidate for 2028. He has been held at the Marmara prison complex ever since, where his trial is due to take place.

His detention triggered the country's largest street protests in more than a decade, followed by hundreds of arrests and a police crackdown.

Apart from the corruption case, prosecutors have accused him of a raft of other offences including espionage and forging his university degree, a qualification he would need to become president and that has since been annulled.

Turkish authorities deny that the judiciary is being used as a political tool.

But the trial has been strongly criticised by rights groups.

"The trial of Mayor Ekrem Imamoglu follows more than a year of weaponising the criminal justice system against his party and other CHP elected officials while he sits in jail," Benjamin Ward, deputy Europe and Central Asia director at Human Rights Watch, said in a statement last week.

"Looking at these cases as a whole, it's hard to avoid the conclusion that prosecutors are trying to remove Imamoglu from politics and discredit his party in ways that undermine democracy."

Amnesty International called the charges brought against Imamoglu "absurd".

"This politically motivated prosecution, which is based almost entirely on secret witness testimony, is riddled with serious international fair trial and rule of law issues," Dinushika Dissanayake, Amnesty's deputy director for Europe, said on Friday.

The Turkish government has insisted that the judiciary operates independently.

Justice Minister Akin Gurlek, the former chief prosecutor who led the investigation into Imamoglu, told reporters on Friday: "I simply did my duty as a public prosecutor. My conscience is clear."

Imamoglu was first elected as Istanbul's mayor in 2019 and was re-elected in April 2024, defeating the governing AK Party candidate by almost a million votes.

His victory stunned Erdogan and his party, with many political commentators in Turkey describing it as the president's "worst defeat ever".