Image copyright Getty Images Image caption The business leaders are making rare public appearances for the questioning

The woman accused of being behind a massive corruption scandal in South Korea has been ordered to appear before a parliamentary hearing.

Choi Soon-sil has been charged over allegations she colluded with President Park Geun-hye to gain influence and money for herself.

Ms Park, who denies corruption, faces an impeachment hearing on Friday.

Amid ongoing street protests, she has said she will resign once parliament finds a way for her to do so smoothly.

The parliamentary hearing in Seoul is questioning the heads of some of South Korea's biggest companies, including Samsung, Lotte, Hyundai, SK and LG.

All the companies gave large donations to foundations run by Ms Choi. They are being quizzed over whether the donations were used to gain them favourable treatment by the government.

Image copyright AFP Image caption Choi Soon-sil, the woman at the centre of the scandal, is in custody awaiting trial

All have denied improper activity, though suggested there had been pressure to make donations.

One of the corporate bosses acknowledged that it was difficult for firms to say no to government requests.

"It's a South Korean reality that if there is a government request, it is difficult for companies to decline," said Huh Chang-soo, head of the GS Group and chairman of the Federation of Korean Industries lobby group.

Ms Choi, who is in police detention along with two other aides of Ms Park, has so far refused to attend the hearing as a witness, citing ill health.

But on Wednesday morning she and several other key witnesses, including members of her family, were ordered to appear.

"This hearing is being criticised as a Choi Soon-sil trial without Choi Soon-sil," said the committee chairman Kim Sung-tae, according to the Agence France Presse news agency.

He said the panel would "undertake all measures" to make her and other witnesses appear, before sending security officers to collect the group.

They could face jail or fines if they refuse.

The extraordinary scenes are being broadcast live on TV. The panel has no power to punish but its chairman has said the hearing is a place for apologies.

Ms Park has apologised multiple times to the public for allowing Ms Choi inappropriate access to government decisions but has stopped short of resigning.

Last week she said she would leave it to parliament to decide her fate, and on Tuesday she was quoted by her party's leader as saying she would accept the outcome of Friday's impeachment vote.