South Korea's former president Moon Jae-in claimed Friday that prosecutors abused their power, were politically motivated and "unjust" to indict him on bribery charges.

Moon, known for his dovish approach to North Korea and peacemaking efforts, was indicted Thursday in a case linked his ex-son-in-law's employment at a Thai airline.

The case adds another layer to the political turmoil engulfing South Korea, where snap elections had to be called following Yoon Suk Yeol's impeachment for declaring martial law.

Former president Moon, 72, is accused of accepting a bribe of $151,000 through his former son-in-law's employment at the airline, along with the salary and living expenses provided by it. 

The company, low-cost airline Thai Eastar Jet, was controlled by a former MP from Moon's party, who was allegedly seeking to gain favour with the then-president.

But during a televised meeting with the National Assembly Speaker on Friday, Moon said that "not only is the indictment itself unjust, but I also felt that the prosecution was just pushing it through in a pre-determined direction, no matter what."

He called the case "a very clear exmaple of the prosecution being politicised".

According to prosecutors, Moon's ex-son-in-law was appointed managing director of Thai Eastar Jet "despite lacking any relevant experience or qualifications in the airline industry".

They added that he frequently took extended leave and handled only basic tasks, such as sending emails was was unemployed before joining the airline.

Political revenge is a recurring theme in South Korea's landscape.

The country's only two other surviving former presidents, Lee Myung-bak and Park Geun-hye, have both been found guilty of corruption and spent time behind bars.

Prosecutors are also currently investigating a stock price manipulation case allegedly involving former first lady Kim Keon Hee, the wife of conservative ex-president Yoon. 

Moon, who while in office brokered talks between Pyongyang's leader Kim Jong Un and US President Donald Trump during his first term, remains an influential figure in his liberal-leaning Democratic Party (DP).

Lee Jae-myung, currently the DP's strongest presidential candidate, is leading the race by a double-digit margin over contenders from ex-president Yoon's scandal-tainted conservative People Power Party (PPP).