Denis Sassou Nguesso has been re-elected president of Congo-Brazzaville after the constitution was amended to allow him to seek a third term.
He won 60% of the vote, against 15% for his nearest challenger, the electoral body announced.
The opposition says there was “massive fraud”.
It says the communications blackout announced during the vote was designed to prevent observers from monitoring the election.
Mr Sassou Nguesso has been in power since 1979, except for a five-year period after losing elections in 1992.
The constitution was changed after an October referendum to remove term and age limits which would have prevented the 72-year-old from standing again.
Riot police fired tear gas at opposition activists who complained about alleged irregularities after voting ended on Sunday in the capital, Brazzaville.
Guy-Brice Parfait Kolelas got 15% of the vote, while General Jean-Marie Michel Mokoko came third with 14%, the electoral commission said.
Before voting day, Congolese Interior Minister Raymond Mboulou had ordered telephone operators to cut all communications – telephone, text messages and the internet.