Ousted South Korean leader Park Geun-hye faced criticism on Monday over a defiant vow that the truth of her impeachment would be revealed, with the main opposition party urging prosecutors to investigate her quickly.

The Constitutional Court dismissed Park from office on Friday when it upheld a parliamentary impeachment vote over an influence-peddling scandal that has shaken the political and business elite. Park has denied any wrongdoing.

On Sunday evening, Park left the Blue House presidential palace in Seoul to return to her private home in the city as an ordinary citizen, stripped of her presidential immunity that has shielded her from prosecution.

"Even at the moment she left, she refused to say a word to repent in front of the people, but said such and such about truth and declared nothing but disobedience," Choo Mi-ae, head of the biggest opposition party, the Democratic Party, told a meeting on Monday.

Park has not commented publicly since the Friday court ruling, but a spokesman read out a statement from her after she returned to her home in the upmarket Gangnam district, in which she expressed regret at not being able to complete her term.

"It will take time, but I believe the truth will be revealed," Park said through the spokesman.

Many South Koreans have interpreted that as a protest against the Constitutional Court's ruling against her.

"It was very shocking and regrettable," said Yoo Seong-min, a presidential candidate from the small, right-wing Bareun Party.

"Protesting the constitutional court's decision is a betrayal of the people and betrayal of the constitution."

'SWIFT AND THOROUGH'

Choo said Park should be treated as a suspect and cooperate with any investigation.

"Prosecutors must find the truth and punish any crime through their swift and thorough investigation," Choo said.

Prosecutors accused Park of colluding with a friend, Choi Soon-sil, to pressure big businesses into contributing to foundations set up to support her policy and allowing her to exert influence on state affairs.

Park and Choi both denied wrongdoing and Park declined to answer prosecutors' questions.

Park, 65, is South Korea's first democratically elected leader to be forced from office.

A snap presidential election will be held by May 9. Moon Jae-in of the Democratic Party is favourite in opinion polls.

Park's dismissal followed months of political paralysis and turmoil over the scandal that also landed the head of the Samsung conglomerate in jail and facing trial.

The crisis has coincided with rising tension with North Korea and anger from China over the deployment in South Korea of a U.S. missile-defence system.

Parliament set up team of special prosecutors to investigate the scandal but its mandate has run out and the case has been handed over to regular state prosecutors.

The Seoul Central District Prosecutors' Office was not available for comment but media reported that prosecutors were considering summoning Park as early as this week.