Zambia's President Michael Sata has died in London while being treated for an undisclosed illness, the government says.

"As you are aware, the president was receiving medical attention in London," Ronald Msiska told state television on Wednesday.

"The head of state passed on Oct. 28. President Sata's demise is deeply regretted. The nation will be kept informed on burial arrangements," he said. "I urge all of you to remain calm, united and peaceful during this very difficult period."

Sata, 77, was a longtime opposition leader before becoming president in 2011. He died late Tuesday at London's King Edward VII hospital.

The Associated Press says: "Rumors that Sata was deathly ill had gripped Zambia since the leader largely dropped out of public view months ago, and opposition groups had questioned whether Sata was fit to lead a country of 15 million people that has enjoyed robust economic growth but suffers widespread poverty."

Al-Jazeera writes of Sata:

"Detractors, political foes, the media and even allies frequently came under attack from a man who earned the sobriquet 'King Cobra.'"He once publicly upbraided his whole Cabinet, threatening to collapse his own government if they did not do a better job."The final period of Sata's rule saw a crackdown on political opponents and critical journalists who reported on his long-suspected illness and frequent 'working trips' abroad, apparently for medical treatment."

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