KINSHASA, Congo — After an election marred by missing ballots and violence, officials extended voting to a second day Tuesday in an attempt to prevent further unrest in sub-Saharan Africa's largest nation.

Country experts had urged the government to postpone Monday's presidential and legislative elections, arguing that a delayed vote was better than a botched one.

Congo is in a race against the clock, though, because the five-year term of President Joseph Kabila expires next week, and the country could face more unrest if he is seen as staying past his constitutional mandate.

The vote is only the second since the end of Congo's last war, and the first to be organized by the government instead of the international community. The election was supposed to mark another step toward peace, but if the results are not accepted by the population, especially the country's fractured opposition, analysts fear it could drag Congo back into conflict.

The spokesman of the election commission, Matthieu Mpita, announced late Monday that the election would be extended into a second day.

"Voters at polling stations that never received ballots and which have not yet opened should await the delivery of the materials," he said. "Voters that are at sites where ballots ran out and where the vote had to be interrupted for whatever reason are asked to stay calm and await further instructions."

Less than 2 percent of roads are paved in Congo, which suffered decades of dictatorship and two civil wars. Some districts are so remote that ballot boxes had to be transported across muddy trails on the heads of porters, and by dugout canoe across churning rivers.

Even in the capital, though, one precinct ran out of ballots late Monday and had to call for more to be delivered, said Jean-Felix Dikamba, the president of one of the polling stations inside the school.

The ballots were delivered in an unmarked car and when the poll workers tried to unload the materials, a mob rushed the car, accusing the poll workers of delivering pre-marked ballots. Police then fired tear gas to disperse angry voters outside.

Elsewhere, five people were killed in the southeastern town of Lubumbashi on Monday after gunmen opened fire on a truck carrying ballots and on a polling center.

The head of the United Nations peacekeeping mission in Congo, Roger Meece, told reporters that he had received reports of at least two polling stations being set on fire in the Kananga province.

Congo's territory straddles an area the size of the United States east of the Mississippi — over 1.4 million square miles, much of it covered by rain forest. The vast forest in the country's east is still inhabited by militias and rebel groups responsible for attacks villages and raping civilians.

The incumbent president is widely expected to win re-election since the opposition is split among 10 candidates, including 79-year-old Etienne Tshisekedi, a longtime opposition leader who is running for president for the first time.

Kabila was first thrust into the position of president a decade ago, after the assassination of his father, Laurent Kabila, the rebel leader who toppled the country's dictator of 32 years, Mobutu Sese Seko.