President of Namibia Hage Gottfried Geingob speaks during the 75th anniversary celebrations of the United Nations Educational, Scientific and Cultural Organization (UNESCO) at UNESCO headquarters in Paris, France, November 12, 2021. JULIEN DE ROSA/ POOL VIA REUTERS

Hage Geingob, President of Namibia, one of Africa's most stable democracies, died Sunday while receiving medical treatment at a local hospital, his office announced.

The Namibian presidency said Geingob's medical team at Lady Pohamba Hospital did its best to help him, but he died with his wife, Monica Geingos, and children by his side, in a post on X, formerly Twitter,

Angolo Mbumba, Namibia's acting president, called for calm, saying in the same post that the "Cabinet will convene with immediate effect in order to make the necessary state arrangements in this regard."

Local media reported Mbumba has called for an urgent cabinet meeting.

According to Namibia's constitution, there should be an election to choose a new president within 90 days of Geingob's death.

Geingob was undergoing treatment for cancer. The 82-year-old had a colonoscopy and a gastroscopy on Jan. 8, followed by a biopsy, his office said last month.

He returned home on Jan. 31 from the United States where he had undergone a trial two-day "novel treatment for cancerous cells," according to his office. In 2014, he said he had survived prostate cancer.

Geingob, president of the southern African nation since 2015, was set to finish his second and final term in office this year. He was the country's third president since it gained independence in 1990, following more than a century of German and then apartheid South African rule.

After spending nearly three decades in exile in neighboring Botswana and the U.S. as an anti-apartheid activist, Geingob returned to Namibia as its first prime minister from 1990 to 2002. He also served in the same capacity from 2008 to 2012.

Soft-spoken but firm on advancing Africa's agenda as an important stakeholder in world affairs, Geingob maintained close relations with the U.S. and other Western countries.

But, like many African leaders, he also forged a warm relationship with China, refuting claims that Beijing is aggressively asserting economic influence over countries in Africa as a form of colonialism.

Namibia, which is on the southwestern coast of Africa, enjoys political and economic stability in a region ravaged by disputes, violent elections and coups. However, the country's opposition slammed Geingob last year for endorsing disputed elections in Zimbabwe.

Condolences from various African leaders poured in on Sunday.

Zimbabwe's President Emmerson Mnangagwa posted on X, saying Geingob's "leadership and resilience will be remembered."

Cyril Ramaphosa, president of neighboring South Africa and one of Namibia's largest trading partners, described him as a " close partner in our democratic dispensation" and "a towering veteran of Namibia's liberation from colonialism and apartheid."

Kenya's Prime Minister William Ruto said Geingob was a "distinguished leader who served the people of Namibia with focus and dedication" and "strongly promoted the continent's voice and visibility at the global arena."

In a statement, Russian President Vladimir Putin said he would "forever cherish" his memories of meeting Geingob. "It is difficult to overestimate his personal contribution to developing friendly relations between Namibia and Russia."

Namibia, a country of just over 2.5 million people, is rich in minerals such as diamonds, gold and uranium. Despite being classified as an upper-middle-income country, socioeconomic inequalities are still widespread, according to the World Bank.

Namibians were expected to head to the ballots in November to choose a new leader.