CARACAS, Venezuela—Hugo Chávez, a former tank commander turned populist politician who used Venezuela's oil riches to challenge the U.S. with his fiery brand of socialism, died Tuesday from complications related to cancer. He was 58 years old.

"We have received the hardest and most tragic news," Vice President Nicolás Maduro said in a national television address, his voice breaking and fighting back tears.

With Mr. Chávez just months into his fourth term, his death plunged Venezuela into political uncertainty. The foreign minister said Mr. Maduro, as expected, would take over for Mr. Chávez until an election is held within 30 days. But some in the ruling party disputed that, saying it should be the head of the country's congress, known as the National Assembly. The Supreme Court declined to immediately weigh in on the controversy.

Just hours before announcing the death, Mr. Maduro painted a picture of a country beset by conspiracy from foreign powers, suggesting the U.S. may have caused Mr. Chávez's cancer and kicking out two U.S. military attaches for allegedly trying to destabilize the country. The U.S. rejected the allegations.

Mr. Chávez's death is a blow to populist governments in the region, including those of Bolivia and Ecuador, which he aided economically and led in a perennial campaign against American dominance. His death could have major economic and political repercussions for Cuba, which receives billions in virtually free oil from Venezuela.