SEOUL - Thousands of South Koreans took to the streets Saturday to demand embattled President Park Geun-Hye resign over a crippling corruption scandal.

Protesters chanted slogans and carried banners urging Park to step down, the day after she issued a grovelling televised apology over the influence-peddling affair which has shattered public trust in her leadership.

Some 40,000 demonstrators were expected to turn out at the rally in central Seoul, watched over by 20,000 police officers.

Police had banned protesters from marching through the streets, citing traffic congestion, but thousands were permitted to gather.

But authorities were braced for demonstrators trying to make their way towards the presidential Blue House.

In a highly personal televised address to the nation Friday, Park said the scandal involving her long-time confidante Choi Soon-Sil was "all my fault", but denied reports linking her and Choi to a religious cult.

Park's approval rating crashed to just five percent this week, the lowest-ever rating for a sitting South Korean president.

"I thinks she should step down as she has lost all her moral authority as president", a 53-year-old, who gave only his family name Ahn, told AFP.

A formal investigation is focused on allegations that Choi, 60, leveraged her close relationship with Park to coerce local firms into donating large sums to dubious non-profit foundations that she then used for personal gain.

Choi was formally arrested on Thursday on charges of fraud and abuse of power, but public anger has largely focused on allegations that she meddled in affairs of state and had access to confidential documents, despite having no official position or security clearance.

The South Korean media has portrayed Choi, whose late father was a shadowy religious leader and an important mentor to Park, as a Rasputin-like figure who wielded an unhealthy influence over the president.

"What is really irritating is the fact that Choi was acting like a regent for Park, controlling her decision-making", said a 20-year-old political science student Kim Do-Hyun at the rally.

In an effort to restore trust in her administration, Park has reshuffled ministers and senior advisers, bringing in figures from outside her ruling conservative Saenuri Party.

Reacting to her address, the main opposition Democratic Party insisted her changes had been cosmetic and warned that it would begin a campaign for her ouster unless further steps were taken.

Park is unlikely to step down, with analysts suggesting she will limp on to the end of her term with her power severely undermined at a time of slowing economic growth, rising unemployment and elevated military tensions with North Korea.