Opposition calls for a boycott and lack of materials combined to reduce voting to a trickle at many polling stations in the capital, witnesses said.
Voting started slowly in a referendum in Congo Republic to determine whether 71-year-old President Denis Sassou Nguesso can legally stand for a third consecutive term in next year's election.
Nguesso is the latest long-serving African president to pursue legal action to prolong his grip on power. Several other such bids have provoked violence, and four died in Congo last week when security forces opened fire on protesters.
Opposition calls for a boycott and lack of materials combined to reduce voting to a trickle at many polling stations in the capital, witnesses said. In some places, the only voters were members of the security forces.
"When the material arrived at the polling station during the night there was no one here to receive it, so they were obliged to leave it at the police post," said Gaspar Mbongo, a local election commissioner at one Brazzaville polling station. "This morning the head of the voting bureau had to get the materials back from the police station and this explains the delay."
Analysts have warned of further violence in the country, which is rich in oil. Dozens of residents moved from southern neighbourhoods of Brazzaville to other areas on Saturday to avoid conflict, although several said they were worried they would not be able to vote as a result. Others complained they had not received voter cards.
Nguesso has ruled for 31 of the past 36 years. He won disputed elections in 2002 and 2009, and under the present constitution term limits and his age bar him from running again.
Burkina Faso's leader of 27 years was toppled last October by protests, and the president of Burundi won a third term in July amid violent protests. Rwanda and the Democratic Republic of Congo have also sought constitutional change.
The legal changes offer a challenge for Western governments - they can endorse veteran leaders or press for term limits. Congo is a former French colony and President Francois Hollande said last week Sassou Nguesso had the right to consult his people.