South Korean president Park Geun-hye has denied allegations that she conspired with a longtime friend to extort money and favours from companies, accusing her opponents of framing her.

In a meeting with a selected group of reporters on Sunday, Park denied that she gave her jailed friend, Choi Soon-sil, extraordinary sway over government decisions, South Korea's Yonhap news agency reported.

It was the first time Park spoke to the media since South Korea's opposition-controlled parliament voted on December 9 to impeach her over the scandal, which has seen millions of people protest in recent weeks.

"The matter is under investigation, so I can't make detailed explanations that might put both sides in trouble, but what I can assure you is that I have never conspired with anyone or did anything to give favours to someone, not even by a bit," she said.

The Constitutional Court has up to six months to decide whether Park should be permanently removed from office or be reinstated.

She has denied wrongdoing previously, but apologised for carelessness in her ties with Choi, a friend for four decades, who has also denied wrongdoing.

Park was quoted on Sunday as saying that the decision by the country's national pension fund to back a merger between two Samsung Group affiliates was "a just policy decision" made for national interest, and that the deal was supported by many brokerage firms at the time.

The merger in 2015 of Samsung Group affiliates Cheil Industries Inc and Samsung C&T Corp has become central to the investigation of the scandal that led parliament to vote last month to impeach Park.

Park, 64, is also accused of colluding with Choi to pressure big businesses, including Samsung, to make contributions to non-profit foundations backing presidential initiatives.