Elected on a "no-corruption" ticket, South Korean President Park Geun-hye now looks set to become the country's first sitting president to be formally questioned as a suspect in a criminal investigation.

It's a stunning fall from grace for a politician who had run as an incorruptible candidate, declaring herself "married to the nation".

High-level corruption has long been a stain on South Korean democratic credentials and the presidential Blue House is no stranger to allegations of cronyism.