Gunmen opened fire on a truck loaded with unused ballots that was heading to polling stations in Lubumbashi, around 1,580 km southeast of Kinshasa, according to Dikanga Kazadi, the interior minister of Katanga province.

Later in the day, armed men attacked voting centers in another neighborhood of Lubumbashi, which is the country's second-largest city.

A total of five people were killed in the two attacks — one policeman and four assailants, he said.

Congo's presidential and legislative elections have been marked by massive logistical challenges, especially the late delivery of voting materials. Experts and opposition leaders had asked the government to delay the ballot for fear that if a significant number of people are unable to vote, it will discredit the election.

As of Monday afternoon, polling stations had still not opened in the Kenya neighborhood of Kinshasa because voting materials had not been delivered.

In Massina, another poor district near Kinshasa's airport, anger was beginning to boil over as voters continued to wait outside of still-unopened polling stations.

In the eastern city of Goma, Cindy McCain said that observers had stumbled upon what may be a case of ballot stuffing. The wife of US Sen. John McCain is part of a team of independent observers.

Ballots boxes already had been partly filled when the observers arrived before the 6 a.m. opening time at the Farajada polling station.

"I don't know exactly what we were looking at," said McCain, who explained that when they asked poll workers about the partially filled boxes, they were told the ballots belonged to election officials who voted first.