HARARE, Zimbabwe — Presidential election results showing that the man who ousted Robert Mugabe in a coup last year in Zimbabwe had won his own term in office were “fraudulent and illegitimate,” the main opposition candidate said on Friday as he vowed to challenge them in the courts.

Tension seemed to build just hours after election officials said President Emmerson Mnangagwa, who seized power last fall with the military’s backing, had defeated his main rival, Nelson Chamisa, in Monday’s voting.

Police officers tried to break up Mr. Chamisa’s news conference before it began, entering the Bronte Hotel in the capital, Harare, and ordering a large crowd of journalists to leave. But the government’s information minister, Simon Khaya-Moyo, directed the police to stand down and allow the news conference to proceed.