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.
Yesterday, the two opposition candidates rejected partial results released by the national electoral commission, which gave the country's long-time ruler 67 percent of votes.
"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".
