PORTLAND, ORE—Mohamed Osman Mohamud pleaded not guilty Monday to charges of attempted use of weapons of mass destruction in an alleged plot to detonate explosives last Friday at a Christmas tree-lighting ceremony attended by thousands at Portland's Pioneer Courthouse Square.

Mr. Mohamud spoke softly in an appearance lasting less than 20 minutes at Portland's Federal district court Monday. He was shackled at the ankles, and appeared to be wearing prison-issue trousers and a light-blue button-down shirt. He didn't acknowledge...