Protesters demonstrate in Kinshasa on Dec. 20. Gunfire was heard in Congo's two largest cities as President Joseph Kabila stayed on past the end of his term. (Eduardo Soteras/AFP/Getty Images)

Security forces clashed with protesters in the capital of Congo on Tuesday, leaving up to 20 people dead as a political crisis escalated over the president’s refusal to step down at the end of his formal term.

The demonstrations were scattered across Kinshasa, a sprawling city of 12 million, and were quickly put down by heavily armed soldiers and police. Young men set tires aflame and threw rocks at police, who responded with tear gas. There were few details about the killings, but some hospitals in the city reported receiving people who had been shot or beaten. By midafternoon, police officers could be seen arresting young men in large groups.

The United Nations had “received reports that 20 people were killed,” according to Felix Basse, a U.N. spokesman in Kinshasa. By Tuesday evening, it was still working to confirm those reports, he said.

Monday was originally meant to be President Joseph Kabila’s last day in office, but his coalition claimed that the country lacked the money or logistical resources to hold elections before 2018, a move widely perceived by Congolese citizens and the international community as a way to extend his rule. Opposition leaders encouraged their followers to defy that decision by taking to the streets, where police and soldiers had been deployed en masse to enforce a government ban on public demonstrations. During the last sizable anti-government protests, in September, about 50 people were killed.

“Today we are taking things into our own hands,” said Peter Kabongo, 27, who was preparing to join a demonstration in Kinshasa’s Matete district, where periodic gunfire could be heard. “The police have guns, but there are millions of us who want Kabila out.”

Debris burns during protests in Kinshasa, Congo, on Dec. 20. (John Bompengo/AP)

There were also protests in Lubumbashi, the country’s ­second-largest city, and witnesses reported that demonstrators had set a gas station on fire.

Some experts said Tuesday’s events showed how opponents of the government have become emboldened, even in the face of the heavy security presence.

“This shows us what is ahead,” said Mvemba Phezo Dizolele, a lecturer in African studies at the Johns Hopkins School of Advanced International Studies. “There’s a tremendous discontent that has been subterranean and is now bubbling up to the surface. We’re going to continue to see that emerge.”

The country, also known as the Democratic Republic of the Congo, is sub-Saharan Africa’s largest nation and has not had a peaceful handover of power since it became independent from Belgium in 1960. Kabila has ruled since 2001, when he assumed the presidency after the assassination of his father, President Laurent Kabila.

Most Congolese want Joseph Kabila to step down, according to opinion polls. He has served two terms, and the country’s constitution does not permit leaders to run for a third term. But Kabila argued that the country is not prepared to hold elections, and the Constitutional Court ruled that he could stay on until a new president is chosen. The court is widely seen as being loyal to Kabila.

A number of African leaders have found ways to stay in power for decades, by challenging constitutional limits or winning elections marred by irregularities. Angola’s José Eduardo dos Santos, 74, has ruled for 37 years. Zimbabwe’s Robert Mugabe, 92, has been in power for 30 years.

In this mineral-rich country, power often translates to wealth, and reports have linked Kabila’s family to a large fortune, while most citizens here earn less than $2 a day. Some politicians from other parties have backed his plan to delay elections, but many others, including the country’s most powerful opposition figures, have not.

Crisis negotiations between Kabila’s coalition and the largely fragmented opposition have been unsuccessful. In a video posted Monday night, opposition leader Etienne Tshisekedi called on people to “peacefully resist [Kabila’s] coup d’etat.”

On Monday, Kabila’s chief ­diplomatic adviser, Kikaya Bin Karubi, called anti-government demonstrators a “loud and insistent mob.” He said the protests would have no bearing on Kabila’s decision to stay in power until the delayed elections. Late Monday, the government announced new cabinet appointments, including some opposition figures who agreed to postpone elections until 2018.

The country is still reeling from conflicts between 1996 and 2003 that left millions dead. Experts and Western officials worry that the political instability caused by Kabila’s efforts to stay in power will reignite fighting.

As the day ended, the only people left demonstrating on the streets of Kinshasa were men wearing flags emblazoned with Kabila’s face and the phrase “We support the president.”

The police did not stop them, despite the ban on public gatherings.

“The president gave us this big road,” said Etienne Kayembe, 40, pointing to the city’s main highway, which had emptied amid the massive police presence. “Before Kabila, we had nothing like this.”

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Today’s coverage from Post correspondents around the world

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