Postelection violence in Gabon left one person dead on Thursday after officials declared the incumbent president, Ali Bongo Ondimba, the winner in a race that the opposition said had been marked by fraud. A civilian, who was not identified, died as security forces encircled the party headquarters of the opposition candidate, Jean Ping, early Thursday after hundreds of people had taken to the streets of the capital, Libreville. The protesters had set fires and insisted that Mr. Bongo had stolen the vote to claim a second term in office. Violence surged almost immediately after the release of election results Wednesday night that said Mr. Bongo, whose family has held the presidency since the late 1960s, had narrowly edged out Mr. Ping in voting on Saturday. The military was sent in to quell the demonstrations, aiming tear gas at protesters who were demanding a recount.