Interior minister Raymond Zephyrin Mboulou announced the results on national television at 3:30 am (0230 GMT) following a tense weekend poll held under a communications blackout.
The official count gave Guy-Brice Parfait Kolelas 15 percent of the vote while General Jean-Marie Michel Mokoko came in third with 14 percent.
Oil- and timber-rich Congo has been on edge since an October constitutional referendum that ended a two-term limit on presidential mandates, allowing the 72-year-old former paratrooper colonel to run for office again.
Authorities imposed a communications blackout during the vote to prevent opposition candidates from publishing "illegal results". A government source said they would remain suspended until after the official results were announced.
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"How do you want us to accept such a result?" General Mokoko said, calling for a recount.
Kolelas' spokesman Vivien Manangou said Sassou Nguesso's reelection was part of a "massive fraud", describing the results as "magic".