More than 20 foreigners being detained at an immigration facility in Tokyo have been on hunger strike since earlier this week, protesting against Japanese immigration policy, support groups and officials said Friday.

The hunger strike, which began Tuesday evening, was launched to protest multiple detentions after provisional release and demand an improvement in conditions at the detention facility, according to a support group for detainees.

The Chinese, Myanmar and other foreign nationals said in a letter to the chief of the Tokyo Regional Immigration Bureau that multiple detentions constituted a "serious human rights violation." The detainees also claim they have been subjected to intimidation and blackmail during questioning at the center.

Around 570 foreigners are currently being detained in the regional immigration authority's facility, according to the bureau.

"We want them to exploit the existing appeals system rather than protest through a hunger strike," a bureau official said.

The hunger strike follows an incident in which a Vietnamese man in his 40s died in hospital in March after an immigration official found him unconscious in his detention room at a facility in Ibaraki Prefecture. Reuters reported that the man died of a stroke and had been left lying in the room for hours before being taken to hospital.