South Korean prosecutors said Sunday that they believe President Park Geun-hye conspired in criminal activities of a confidante who allegedly exploited her presidential ties to amass an illicit fortune — a damning revelation that may convince opposition parties to push for Park's impeachment.

Prosecutors want to interrogate Park in person, but presidential spokesman Jung Youn-kuk, who said the investigation resembled a "character assassination" on Park, strongly hinted that the president would continue to refuse their questioning.

Prosecutors on Sunday formally charged the confidante, Park's longtime friend Choi Soon-sil, on suspicion of interfering with state affairs and bullying companies into giving tens of millions of dollars to foundations and businesses she controlled.

In a televised news conference, Lee Young-ryeol, chief prosecutor of the Seoul Central District Prosecutors' Office, said that based on the evidence, "the president was collusively involved in a considerable part of the criminal activities by suspects Choi Soon-sil, Ahn Jong-beom and Jung Ho-sung." He was referring to two presidential aides who also were formally charged Sunday for allegedly helping Choi.

"However, because of the president's impunity from prosecution stated in Article 84 of the constitution, we cannot indict the president," Lee said. "The special investigation headquarters will continue to push for an investigation of the president based on this judgment."

Jung, the presidential spokesman, said details announced by Lee were "entirely untrue, and are just a tower built on sand, based on imagination and guesswork and ignorance of objective facts."

"We do not think that the prosecution's investigation so far has been fair and politically neutral," he said.

Jung said that the president plans to prove her innocence by "actively cooperating" with an independent investigation pushed by a special prosecutor. Park has immunity, but can be investigated.

The country's largest opposition party, the Minjoo, issued a statement Sunday calling for Park to immediately resign, saying that a "criminal suspect" should not be allowed to lead the country.

Park is facing growing calls to resign over a scandal that critics say has undermined the country's democracy. Although emboldened by a wave of mass protests, opposition parties have so far refrained from seriously pushing for Park's impeachment over fears of triggering a backlash from conservative voters and negatively impacting next year's presidential election.

However, there are growing voices within the opposition saying that an impeachment attempt is inevitable because it's unlikely Park will resign and give up her immunity.

On Sunday, eight prominent leaders of the three parliamentary opposition parties gathered before issuing a joint statement calling for their parties to begin discussions on an impeachment push.

Ahn Jong-beom, Park's former senior secretary for policy coordination, was charged with abuse of authority, coercion and attempted coercion over allegations that he pressured companies into making large donations to foundations and companies Choi controlled.

Jung Ho-sung, the other former aide who was indicted, was accused of passing on classified presidential documents to Choi, including information on ministerial candidates.

According to Lee, Choi and Ahn conspired to pressure companies into giving a combined 77.4 billion won ($65.5 million) to the Mir and K-Sports foundations, two nonprofits that were under Choi's control. The companies couldn't refuse because they feared doing so would result in business disadvantages, such as difficulties in gaining government approval for projects or being targeted in tax investigations, Lee said.

Additionally, Choi and Ahn pressured the Lotte Group into giving 7 billion won ($5.9 million) to the K-Sports foundation to finance the construction of a sports facility in the city of Hanam, which was to be operated by The Blue K, a company established by Choi, Lee said. The money was later returned.

Auto giant Hyundai and telecommunications company KT were forced to contract 13 billion won ($11 million) worth of their advertisements to Playground, an ad agency virtually run by Choi, Lee said. Hyundai was also forced to buy 1.1 billion won ($931,000) worth of supplies from an auto parts maker run by Choi's friend. Ahn and Choi also tried but failed to take over the shares of an advertisement company previously owned by steelmaker POSCO, Lee said.

Prosecutors are also seeking to indict Cha Eun-taek, a famous music video director who allegedly used his close relationship with Choi to win lucrative government culture projects, and former vice sports minister Kim Chong, suspected of providing business favors to sports organizations controlled by Choi.

Kim is also under suspicion of influencing the ministry's decision to financially support a sports foundation run by Choi's niece, who prosecutors detained on Friday.

Park, the daughter of slain military dictator Park Chung-hee, first met Choi in the 1970s, around the time Park was acting as first lady after her mother was killed during a 1974 assassination attempt on her father. Choi's father, a shadowy figure named Choi Tae-min who was a Buddhist monk, a religious cult leader and a Christian pastor at different times, emerged as the younger Park's mentor.

The Choi clan has long been suspected of building a fortune by using their connections with Park to extort companies and government organizations. Choi's ex-husband is also a former close aide of Park's.

Park's term lasts until Feb. 24, 2018. If she steps down before the presidential vote on Dec. 20, 2017, an election must be held within 60 days.