NAIROBI, Kenya >> President Pierre Nkurunziza of Burundi died of heart failure Monday just weeks after a crucial election to choose a successor to replace him after 15 years of autocratic rule over the Central African nation. He was 55.

Nkurunziza’s death was announced by the government today on Twitter. He died at a hospital in eastern Burundi after he fell ill over the weekend and was hospitalized after attending a volleyball game.

A former rebel leader, Nkurunziza ruled the country with impunity for years, arresting opponents, stifling media outlets and cracking down on dissent.

His reign drew international condemnation and sparked widespread protests in the tiny country, one of the world’s poorest. The International Criminal Court opened an investigation into whether his government committed crimes against humanity, including murder, torture, rape and disappearances. In 2017, Nkurunziza withdrew Burundi’s membership in the court.

A government spokesman, Prosper Ntahorwamiye, called for calm and announced seven days of mourning during which flags would be flown at half-staff.

“Burundi has just lost an honorable son of the country, a president of the republic, a supreme guide of patriotism,” Ntahorwamiye said in a statement, referring to the official title Nkurunziza would have adopted once he stepped down. His successor was set to be sworn in in August for a seven-year term.

Last month, the secretary-general of the governing party, Evariste Ndayishimiye, won the presidential election after Nkurunziza said he would not seek a new term. Opposition parties and human rights organizations said the vote was marred by irregularities. But Burundi’s constitutional court rejected those claims and last week upheld the election of Ndayishimiye.

Until Ndayishimiye can be inaugurated in August, the president of the national assembly, Pascal Nyabenda, will lead the government, in accordance with the country’s constitution.

Nkurunziza, a former Hutu rebel leader, came to power in 2005 at the end of Burundi’s civil war, during which 300,000 people were killed. Elected by lawmakers, Nkurunziza, a former sports instructor, faced the daunting task of maintaining peace and lasting stability, while bringing economic development to a country facing dire poverty and persistent epidemics.

The landlocked nation of 11 million has struggled to emerge from ethnic conflict and instability since gaining independence from Belgium in 1962.

During his reign, Nkurunziza tightened his grip on power through a campaign of repression. He banned jogging for fear it fed demonstrations. He arrested schoolgirls who doodled on his portrait, and deployed security forces and the ruling party’s Imbonerakure youth league to crack down on political opponents.

The climate of fear in the country continued to rise as Nkurunziza sought a second term in 2010, and a third run in 2015 that his rivals said violated the constitution.

Following a foiled coup attempt that year, turmoil across the country triggered a crackdown by security forces that killed hundreds and caused more than 400,000 others to flee to neighboring countries. A United Nations inquiry into the crisis documented killings, torture and rape committed by security officers, and urged the International Criminal Court to open an investigation.

Nkurunziza withdrew Burundi’s membership in the court in 2017 and shut the U.N. human rights office in the country in 2019. The moves isolated Nkurunziza’s government and pushed donors, like the European Union, to withhold financial support.

In 2018, Burundians voted in a referendum that paved the way for Nkurunziza to stay in power until 2034. But in a surprising about-face, Nkurunziza said he would not run for president again.

His sudden death is likely to have a significant impact on Burundi’s regional and global relations, said Aymar Nyenyezi Bisoka, a professor at the Catholic University of Louvain in Belgium, who studies the African Great Lakes region.

Bisoka said that the president-elect could help mend relations with Rwanda, which have deteriorated in recent years. While Burundi has accused Rwanda of masterminding the thwarted coup in 2015, Rwanda has accused Burundi of sheltering rebels associated with the genocide that ripped Rwanda apart in 1994.

Ndayishimiye might face resistance from the generals and leaders within the ruling party, said Bisoka. But he also has the chance to pitch himself as the man to unite Burundi and lead it out of its political and economic isolation.

“The future of this party and Burundi will depend on Evariste Ndayishimiye’s ability to reassure the generals and key cadres,” Bisoka said in an email interview.

Today, residents of Burundi’s biggest city, Bujumbura, could be seen rushing home, and businesses closed early. Some, like Ntwari Christophe, a university student, said they could not believe the president had died.

Activists said Nkurunziza would be remembered for overseeing a government that crushed dissent and committed human rights abuses.

“His death highlights the urgency for the many victims to know the truth about the crimes committed during his presidency and who was responsible,” said Lewis Mudge, Central Africa director at Human Rights Watch.

“As long as these abuses go unpunished,” he said, “this dark legacy will hang over Burundi for many years to come.”