Taiwan investigators were expected to rule Sunday on the further detention of a former presidential candidate amid graft allegations that have sparked minor street protests by his supporters.

Ko Wen-je, chairman of the Taiwan People’s Party, was held overnight and has a hearing on Sunday, reports AP.

Supporters gathered outside the prosecutors office in Taipei, holding signs and chanting slogans demanding Ko’s release and alleging judicial oppression. Investigators refused Ko's request to return home out of concern he would collude with others involved in the case.

Ko, who trained as a doctor and also served as mayor of Taipei, and his TPP emerged as a third force in a sometimes rambunctious democracy dominated by the ruling Democratic Progressive Party and the remainder of the Nationalist Party that fled to Taiwan after the Communist Party’s takeover of mainland China in 1949.

Ko and others were named in May in the scandal in which the developer of the downtown Core Pacific Center shopping center was given permission to massively expand the floor space available for lease during Ko's 2014-2022 term as mayor. Ko's home and TPP offices were raided before he was called in for questioning on Friday. The shopping center is still under construction and DPP politicians have called for a suspension of work or even demolition.

Ko finished third with about a quarter of the vote in this year's election, which was won by the DPP's William Lai Ching-te, whose party favors continued de-facto independence from China. The TPP won just under 3% of the seats in the legislature.

Ko did particularly well among young people despite having no clear political platform. He flirted briefly with both the DPP and Nationalists but was unable to come to terms on an alliance, leaving him with few political allies outside his own supporters.

Once a major problem, political corruption has declined as a top concern amid a strengthening of the power of independent investigators.