A prominent K-pop music video director has been charged as part of a corruption scandal rocking South Korea and engulfing the country’s president, Park Geun-hye.

Prosecutors claim Cha Eun-taek, who has worked with Gangnam Style star Psy and the boy band Big Bang, used his ties to a secret confidante of Park to win lucrative projects from state agencies and private firms.

That confidante – Choi Soon-sil – has been labelled Park’s éminence grise and is accused of using her close relationship with the president to extract more than $60m (£48m) from top companies including Samsung.

Prosecutors say Park ordered her former economic adviser to help Cha pressure officials and private firms so that he would win contracts.

Cha, 46, has been charged with abuse of power, coercion and embezzlement, and has becomes the latest public figure to be embroiled in the snowballing scandal.

Choi, 60, is accused of meddling in a wide range of state affairs including the country’s preparations for the 2018 Winter Olympics.

Prosecutors last week formally charged her with abuse of power and coercion, saying Park was a “co-culprit” who had colluded with Choi to strongarm top firms into giving cash to non-profit foundations the latter controlled.

Park has rejected a series of requests from prosecutors to answer their questions.

Hundreds of thousands of South Koreans attend a rally in Seoul to demand that President Park Geun-hye step down. Photograph: USA/Rex/Shutterstock

As a sitting president, Park cannot be charged with a criminal offence except insurrection or treason, but she can be investigated and potentially charged once her term is over. She is the first South Korean president to become a criminal suspect while in office.

Park is faced with growing public calls to resign and a push by lawmakers to impeach her, with her job approval ratings diving to record lows of 4%.

Hundreds of thousands of South Koreans have taken to the streets in recent weeks to call for her to step down, with organisers claiming the latest rally on Saturday in Seoul drew 1.5 million people.