Thousands of South Koreans have taken to the streets of the capital Seoul calling for president Park Geun-hye to step down over allegations that she let an old friend, the daughter of a religious cult leader, interfere in important state affairs.

The evening protest came after Ms Park ordered 10 of her senior secretaries to resign over a scandal that is likely to deepen the president's lame duck status ahead of next year's election.

Holding candles and signs reading "Who's the real president?" and "Park Geun-hye step down", the protesters marched through central Seoul after holding a candlelight vigil near City Hall.

Police estimated that about 9,000 people turned out for the biggest anti-government demonstration in Seoul in months.

"Park has lost her authority as president and showed she doesn't have the basic qualities to govern a country," Jae-myung Lee, from the opposition Minjoo Party and the mayor of the city of Seongnam, told the protesters from a stage.

Ms Park has been facing calls to reshuffle her office and cabinet after she acknowledged on Tuesday that she provided long-time friend Choi Soon-sil drafts of her speeches for editing.

Her televised apology sparked intense criticism about her mismanagement of national information and a heavy-handed leadership style that many see as lacking in transparency.

There is also media speculation that Ms Choi, who holds no government job, meddled in government decisions on personnel and policy and exploited her ties with Ms Park to misappropriate funds from non-profit organisations.

Prosecutors on Saturday widened their investigation by searching the homes of presidential officials suspected of interacting with Ms Choi and receiving their office files from the Blue House - the presidential office and residence.

Prosecutors had previously summoned some of Ms Choi's key associates and raided their homes and workplaces, as well as the offices of two non-profit foundations Ms Choi supposedly controlled.

The saga, triggered by weeks of media reports, has sent Ms Park's approval ratings to record lows, and the minority opposition Justice Party has called for her to resign.

The Minjoo Party, a larger opposition party that has refrained from calling for Ms Park's resignation over fears of negatively affecting next year's presidential election, said Ms Park's decision to shake up her secretariat was too little, too late, and called for stronger changes, including the reshuffling of her cabinet.

Ms Park's aides on the way out include Woo Byung-woo, senior presidential secretary for civil affairs, and Ahn Jong-beom, senior secretary for policy co-ordination.

Lee Won-jong, Ms Park's chief of staff, tendered his resignation on Wednesday.

Mr Woo has been blamed for failing to prevent Ms Choi from influencing state affairs and has also been embroiled in separate corruption allegations surrounding his family.

Mr Ahn, whose home was searched by prosecutors on Saturday, is under suspicion that he helped Ms Choi pressure South Korean companies into making large donations to the Mir and K-Sports foundations, launched in October last year and January this year, respectively.

Ms Choi reportedly masterminded the creation of the two non-profits, which managed to gather around 70 million dollars (£57 million) in corporate donations over a short period of time, and is suspected of misappropriating some of the funds for personal use.

Ms Park's office said she plans to announce a new line-up of senior secretaries soon.

Ms Choi's lawyer Lee Gyeong-jae said that she was currently in Germany but would return to South Korea if prosecutors summon her.

In an interview with a South Korean newspaper earlier in the week, Ms Choi acknowledged receiving presidential documents in advance, but denied intervening in state affairs or pressuring companies into donating to the foundations.

Ms Choi and Ms Park reportedly became friends in the 1970s, when Ms Choi's late father, Choi Tae-min, a shadowy religious figure who was a Buddhist monk, cult leader and Christian pastor at different points in his life, emerged as Ms Park's mentor.

At the time, Ms Park was serving as acting first lady after her mother was killed in 1974 by a man trying to assassinate her father, military strongman Park Chung-hee, who would be murdered by his own spy chief five years later.