A Turkish prosecutor has demanded a prison sentence of more than 2,000 years for jailed Istanbul Mayor Ekrem Imamoglu for allegedly leading a vast corruption network that cost the state billions of lira, according to an indictment seen by Reuters.

Imamoglu, who is President Tayyip Erdogan's main political rival undefined, has previously denied all the accusations against him and said they are politically motivated undefined.

Istanbul Chief Prosecutor Akin Gurlek announced the indictment at a press conference on Tuesday, saying it names 402 suspects, including Imamoglu, and accuses them of forming a criminal organisation, bribery, fraud and bid-rigging.

Gurlek said the network caused 160 billion lira ($3.8 billion) in losses to the Turkish state over a 10-year period.

The more than 4,000-page indictment includes an organisation chart that portrays Imamoglu as the founder and head of the criminal group.

It cites findings by the Financial Crimes Investigation Board (MASAK), expert analyses, and digital and video evidence, and alleges that several businesspeople were coerced into paying bribes through a secret fund operating within the municipality.

Reuters could not independently verify the allegations beyond the contents of the indictment. The Istanbul municipality and Imamoglu's lawyers did not immediately respond to requests for comment.

The mayor has been in jail since March pending trial on corruption charges and received a separate prison sentence in July for insulting and threatening the city’s chief prosecutor, a verdict he is appealing.

The government denies the assertion by Imamoglu and his Republican People's Party (CHP) that the case against him is politically driven and says that Turkey's courts are independent.