Thousands of opposition supporters were to take to the streets on Sunday to demand President Chen Shui-bian resign after his wife was indicted on corruption and forgery charges, officials said.
Wu Shu-chen is charged with illegally claiming personal expenses from state funds in the high-profile scandal, which has shaken the ruling Democratic Progressive Party (DPP).
Chen is also suspected of graft and forgery, but as president he is immune from indictment until he leaves office.
Legislator Wu Yu-sheng from the main opposition Kuomintang (KMT) party said they expected thousands of people would protest in Taipei to demand Chen's resignation.
"We expect some 20,000 to 30,000 people would attend the rally," Wu said.
"The evidence gathered by prosecutor Chen Jui-jen showed the president and his wife Wu Shu-chen were involved in the graft and corruption case. Therefore the president must step down," Wu told AFP.
The rally, due to kick off at 0600 GMT, will be led by KMT chairman Taipei Mayor Ma Ying-jeou.
Prosecutor Chen rocked the island on Friday when he said he had enough evidence against Wu following a 100-day investigation. The corruption charge carries a minimum seven-year prison term.
According to the indictment, Wu claimed various personal expenses from state funds allocated to her husband, including at least one million Taiwan dollars-worth (30,400 US) of diamond rings and other items for her son, daughter, son-in-law and grandchildren.
Local media said Chen, under mounting pressure to quit, would make a televised speech Sunday night to defend himself, but the Presidential Office said that as of now, there was no scheduled time for such a speech.
As Chen has show no signs of bowing to pressure, the KMT plans to launch another motion in parliament to force a referendum on the future of the embattled leader. Two previous attempts were aborted.
The recall motion aimed at ousting Chen would require approval of at least two-thirds of the 220 legislators, or 147 votes.
In the second attempt last month, only 116 cast ballots in favor while all 85 lawmakers from the ruling DPP boycotted the vote.
However, the DPP's ally, Taiwan Solidarity Union which has 12 legislators who cast invalid ballots last month, has said they would vote for the third attempt.
That means the recall motion could pass if some 15 to 20 DPP legislators vote against Chen.
KMT legislator Wu urged DPP lawmakers to vote against Chen.
"I really hope DPP legislators would take this matter into serious consideration not from Chen's point of view. Rather this should be weighed from Taiwan's point of view. Corruption and graft should be eliminated from Taiwan," Wu said.
"Drawing a distance from Chen would also benefit the party because it would show the DPP has not forsaken its ideals of building up clean politics for the country," he said.
The DPP on Saturday submitted First Lady Wu and three other party members indicted in the case to its central review committee for disciplinary punishment following a crisis meeting.
But party reformers were forced to agree not to send the president for disciplinary review following strong opposition from his supporters to prevent a rift in the party, local newspapers said.