Former vice-president Annette Lu and Democratic Progressive Party (DPP) chairwoman Tsai Ing-wen are among thousands of people expected to join the late night rally in a park, they said.

Protesters will demand justice for Chen, who has gone on hunger strike to protest what he says are politically motivated corruption allegations against him.

Chen, who stepped down in May as president after serving the maximum eight years, still has a large following despite a string of scandals implicating him, his family and former top officials from his administration.

The ex-leader was hospitalised earlier this week for dehydration, days after going on hunger strike to protest his detention.

Chen's lawyer is planning to bring his case to the island's grand justices after the High Court ruled an appeal against the ex-leader's detention invalid.

Chen's lawyer submitted the appeal despite the ex-president's refusal as he felt such action was in his client's best interests.

Chen, whose pro-independence stance in office set him against Beijing, has repeatedly accused the island's China-friendly government of being behind allegations of embezzlement, money laundering, taking bribes and forgery.

He is the first former Taiwanese leader to be arrested, and could be held for up to four months before prosecutors have to charge him. -- AFP