South Korea's Constitutional Court held its first preparatory hearing Thursday in the trial of impeached President Park Geun-hye, whom lawmakers voted to remove over an explosive corruption scandal that saw millions of people protest in past weeks.

The court confirmed it will hear allegations that Park colluded with longtime confidante Choi Soon-sil to extort money and favors from major South Korean companies and allowed Choi to interfere with government affairs from the shadows.

The court also said it would review accusations in the impeachment bill that Park was responsible for media restrictions and government inaction during a 2014 ferry sinking that killed more than 300 people, mostly teenagers on a school trip.

Choi Soon-Sil, pictured here at her criminal trial at the Seoul Central District Court. Pool / Getty Images

After discussions with prosecutors and Park's lawyers, court decided to summon as witnesses Choi and two former presidential aides. Both were arrested earlier for allegedly helping Choi extort from companies and passing her confidential government information.

The court has up to six months to decide whether Park should permanently step down or be reinstated. Her presidential powers are suspended until then, with the prime minister assuming the role of government caretaker. The next preparatory hearing in Park's trial is scheduled on Dec. 27.

Lawmakers, who have been grilling key suspects to the scandal in weekly hearings broadcast on live TV, plan to take their hearings Monday to two detention centers where Choi and the two former presidential aides are being held in a desperate effort to question them. The three suspects have refused to attend the hearings held at the National Assembly, citing health and other reasons.

South Korea's opposition-controlled parliament voted to impeach the president on Dec. 9 as the scandal grew. The investigation has widened with a special prosecutor since Wednesday conducting raids at the offices of the national pension fund and trying to detain Choi's daughter, Yoora Chung, who is believed to be in Germany.

Choi is suspected of exploiting her presidential ties to get Chung, an equestrian athlete, into an elite university despite questionable qualifications. Lee Jae-yong, the scion of Samsung, South Korea's largest business group, has apologized over the use of corporate funds to buy a horse for Chung, but denied that Samsung sought favors from Choi or Park's administration.

After a request from the investigators, Seoul's Foreign Ministry said it will soon order Choi's daughter to hand in her passport. It will be invalidated if she does not return it, ministry's spokesman Cho June-hyuck said.

An official from the investigation team led by special prosecutor Park Young-soo earlier said it had obtained a warrant to detain Chung and had asked for help from German prosecutors in finding Chung and obtaining evidence, such as financial transaction and phone records. A German prosecution official told South Korean media that Germany will cooperate with South Korean investigators.