Ecuador and Cuba both announced three days of national mourning to honor Chavez.

"The national government expresses its solidarity in light of this irreparable loss that puts the Venezuelan people and all the region in mourning and at the same time sends its heartfelt condolences to the family of the late champion of Latin America," Ecuador's foreign ministry said in a statement.

Bolivian President Evo Morales' voice cracked as he spoke to reporters, describing Chavez as someone "who gave all his life for the liberation of the Venezuelan people ... of all the anti-imperialists and anti-capitalists of the world."

But longtime critics of the controversial president offered a different take.

"Hugo Chavez was a tyrant who forced the people of Venezuela to live in fear. His death dents the alliance of anti-U.S. leftist leaders in South America. Good riddance to this dictator," said U.S. Rep. Ed Royce, chairman of the House Committee on Foreign Affairs. "Venezuela once had a strong democratic tradition and was close to the United States. Chavez's death sets the stage for fresh elections. While not guaranteed, closer U.S. relations with (this) key country in our Hemisphere are now possible."

Venezuela-U.S. relations surge into spotlight

Just hours before the announcement of Chavez's death, relations between the two countries appeared to be souring, as Venezuelan officials said they were expelling two U.S. Embassy officials and accused them of plotting to destabilize the country.

The U.S. officials, both air attaches at the U.S. Embassy in Caracas, are accused of having meetings with members of the Venezuelan military and encouraging them to pursue "destabilizing projects," Venezuelan Foreign Minister Elias Jaua said.

"We will not allow any foreign interference in our country," Jaua said. "Do not think that the situation of pain over the health of President Chavez will translate into weakness."

State Department spokesman Patrick Ventrell denied the accusations.

"Notwithstanding the significant differences between our governments, we continue to believe it important to seek a functional and more productive relationship with Venezuela based on issues of mutual interest," he said. "This fallacious assertion of inappropriate U.S. action leads us to conclude that, unfortunately, the current Venezuelan government is not interested an improved relationship."

After announcing the expulsion of one attache, Maduro -- addressing the media in a lengthy statement -- asserted that someday there will be "scientific proof" that Chavez was somehow infected by outsiders.

"An assertion that the United States was somehow involved in causing President Chavez's illness is absurd, and we definitively reject it," Ventrell said.

It isn't the first time that a Venezuelan government official has implied that a plot could be behind Chavez's cancer.

Chavez made the assertion himself in 2011, saying at a military event in Caracas that he wondered whether the United States could be infecting Latin American leaders with the illness.