South Korean prosecutors are seeking an arrest warrant for the leader of the country's main opposition party over allegations of corruption when he was a city mayor.

The Seoul Central District Prosecutors' Office on Thursday filed the request for Democratic Party leader Lee Jae-myung.

The allegations involve dealings when Lee was mayor of Seongnam, a city south of Seoul, from 2010 to 2018.

It is extremely rare for prosecutors to request an arrest warrant for the head of the largest opposition party. South Korea media reported the story as breaking news.

Lee's Democratic Party has denounced the prosecution's actions as unprecedented and outrageous.

Lee has been charged with breach of trust for allegedly causing about 490 billion won, or about 380 million dollars, in damages by allowing a real estate developer to illegally profit from an urban development project.

Lee is also suspected of providing favors to companies who made sponsorship payments to a local professional soccer club.

Prosecutors have questioned Lee on three occasions. He has denied any wrongdoing.

The Democratic Party has accused the prosecution of trying to nullify the opposition and remove the president's political enemy.

Prosecutors need the consent of parliament to arrest a lawmaker while it is in session, and a motion seeking consent for the arrest will soon be filed.

But South Korean media say the motion is unlikely to be passed because the main opposition party has a majority in parliament.