The flamboyant 58-year-old leader had undergone four operations in Cuba for a cancer that was first detected in his pelvic region in mid-2011. His last surgery was on December 11 and he had not been seen in public since.
"It's a moment of deep pain," Maduro, accompanied by senior ministers, said, his voice choking.
Chavez easily won a new six-year term at an election in October and his death will devastate millions of supporters who adored his charismatic style, anti-U.S. rhetoric and oil-financed policies that brought subsidized food and free health clinics to long-neglected slums.
Detractors, however, saw his one-man style, gleeful nationalizations and often harsh treatment of opponents as traits of an egotistical dictator whose misplaced statist economics wasted a historic bonanza of oil revenues.
Chavez's death opens the way for a new election that will test whether his socialist "revolution" can live on without his dominant personality at the helm.
The vote should be held within 30 days and will likely pit Maduro against Henrique Capriles, the centrist opposition leader and state governor who lost to Chavez in the October election.
One recent opinion poll gave Maduro a strong lead.
Maduro is Chavez's preferred successor, enjoys support among many of the working class and could benefit from an inevitable surge of emotion in the coming days.
But the president's death could also trigger in-fighting in a leftist coalition that ranges from hard-left intellectuals to army officers and businessmen.
Venezuela has the world's largest oil reserves and some of the most heavily traded bonds, so investors will be highly sensitive to any signs of political instability.
A defeat for Maduro would bring major changes to Venezuela and could also upend its alliances with Latin American countries that have relied on Chavez's oil-funded largesse - most notably with communist-led Cuba, which recovered from financial ruin in the 1990s thanks largely to Chavez's aid.
Chavez was a garrulous figurehead for a global "anti-imperialist" alliance stretching as far as Belarus and Iran, and he will be sorely missed by anti-U.S. agitators.