A prominent Nigerian asylum seeker and activist is being held in solitary confinement at a Tokyo detention center, a case that has highlighted a growing crackdown on foreign nationals living in Japan without visas, prompting demands for her release.

Elizabeth Aruoriwo Obueza was detained last month after authorities turned down an appeal against her asylum rejection, Obueza and her lawyer said.

Obueza, 48, campaigns for asylum seekers and the 4,700 people on "provisional release" from immigration detention — a status that lets foreigners out from detention but bars them from working and traveling freely.