Illegal mainland immigrants fighting Tuesday's ruling started a new 24-hour sit-in and threatened to go on a hunger strike if the Government tried to deport them. More than 150 immigrants and their relatives camped out in front of Central Government Offices. The mainlanders, who had overstayed their visitation permits, are demanding the release of 179 fellow overstayers arrested after the ruling. Representatives of the 1,200 overstayers voted unanimously to stage a hunger strike if any of the overstayers were repatriated. Immigration officials said they would not deport anyone pending the appeal. But they pressed ahead with action against mainlanders remaining on temporary permits which required them to report to the Immigration Department every two weeks. Officials continued to arrest overstayers as they reported to the department.