Charles Hyder, a 56-year-old physicist who said he was on the 81st day of a fast to prevent worldwide nuclear holocaust, held a news conference yesterday in Lafayette Park to call for a national day of fasting on Christmas Day.

Hyder, dressed in layers of winter clothes and a knit hat, was applauded by about 35 supporters when he stood to speak, his voice strong but his face gray and lined.

Hyder said that although he had announced earlier that he expected to be dead by Christmas, he now expects to live until the first of the year. He said he has 100 of his original 300 pounds. People of normal weight would have died by now from the kind of strict fasting Hyder described, according to experts.