Activist group the Centre for Military Human Rights Korea (CMHRK), citing multiple leaks, said top military leaders had studied plans to crack down on demonstrators calling for Park's resignation over a corruption scandal.

South Korea was ruled by military dictators for decades -- one of them Park's own father Park Chung-hee -- and only fully embraced democracy in the 1990s, making such issues highly sensitive.

"The defence ministry will immediately launch a probe," the ministry said in a statement.

"We will announce the results of the investigation in a transparent manner and take follow-up measures", it added.

According to the CMHRK the plans were only set aside after the country's top court endorsed the National Assembly's decision to impeach Park.

The former president has been in custody since May and is awaiting a verdict after prosecutors called for 30 years in prison.

"The presidential Blue House, the military and judicial authorities were planning a pro-Park coup," the CMHRK said in a statement.

It would have been "another bloody crackdown on protestors", it said, after the 1980 Gwangju massacre in which some 200 pro-democracy protestors were killed by martial law troops.

The CMHRK called for a thorough investigation and punishment for those responsible, including the then defence minister Han Min-koo and a former commander of the capital's defence garrison.