Embattled President Park Geun-hye is fighting for her political career as opponents call for her impeachment. Moon Jae-in, who served as the opposition leader of the Minjoo Party of Korea said: "Park deserves to be arrested and it's a shame."

Speaking after a meeting with other opposition political figures he added: "We have a strong reason to impeach her. She has to discuss with lawmakers how to exit orderly first. We will help her resign with honour and keep that honour even after her exit if she steps down now."

Prosecutors said in a statement on Sunday (20 November) that the president "colluded" with former aides in a corruption case. They say there is enough evidence that she played a role while her close confidante Choi Soon-sil gained access to classified information and extracted money from some of South Korea's largest companies, including electronics company Samsung and Hyundai.

Charges against Choi and a former presidential aide include attempted coercion and abuse of authority, Lee Young-reyol, head of the Seoul Central District Prosecutors' Office, said on Korean TV.

Park is the first South Korean president to be investigated "as a suspect" in a corruption investigation while in office, said CNN. Her approval rating has dropped by 5%, according to Gallup Korea.

The South Korean president is immune from prosecution due to the country's laws. According to the South Korean constitution, the president cannot be charged with a crime while holding office, except for treason or insurrection.

However, Park's impeachment is possible, but it requires the approval of two-thirds of the 300-member National Assembly.

"Opposition will now start contacting ruling-party lawmakers to start their impeachment procedure," Choi Chang-ryul, a political commentator and professor at Yong In University told Bloomberg. "While it's the prevailing view that any impeachment motion can be overturned by the Constitutional Court, I personally think it won't be that easy for the judges to do so given the public anger."

In an attempt to hold on to power, Park has made tearful apologies to the nation. "These latest developments are all my fault and were caused by my carelessness," she said, adding that she had "allowed my guard to drop" around Choi, her friend for more than four decades.

"It is hard to forgive myself and sleep at night because of the feelings of sorrow," Park said.

She ordered the resignation of 10 of her senior secretaries. She also fired the prime minister, Hwang Kyo-ahn, and appointed Kim Byong-joon, an academic from Seoul's Kookmin University.

South Koreans have called for Park's resignation as local media reported that around 150,000 gathered for a rally in Seoul on Saturday – the fourth so far. One demonstrator, Han Jin-wook, told a reporter, "I wanted to show my children the power of citizens… Some media reported the rally would be violent, but I feel totally safe."