The government invited foreign leaders to add political weight to Thursday's event, and they filled a stage in front of the presidential palace as Maduro addressed the crowd and called it a "historic event."
A recording of Chavez singing the national anthem suddenly appeared, and his followers sang along. At the end, his voice boomed: "Long live the Bolivarian Revolution!"
Sitting beside Maduro were presidents including Daniel Ortega of Nicaragua, Evo Morales of Bolivia and Jose Mujica of Uruguay. The government said officials from about 20 other nations were on hand.
Argentine Foreign Minister Hector Timerman said President Cristina Fernandez was traveling to Havana on Thursday to see Chavez.
Some raised the possibility of Chavez's death, though no one uttered the word.
"There's a man who's fighting a battle for his life, who is in all of your hearts," Mujica told the crowd. "But if he isn't here tomorrow, unity, peace and work, dear friends."
While the visiting leaders spoke, fighter jets thundered overhead, flying low. Members of the Cabinet waved to them, and the crowd went wild waving flags. One of the warplanes did a roll.
Militia troops in fatigues stood in formation. Soldiers and police guarded street corners while hip-hop artists performed in the morning on stages set up along the avenue.
The mood was festive as Chavez's followers blew horns and held up posters reading: "Now with Chavez more than ever."
"We came to show support, so he knows his nation is with him," said Anny Marquez, a secretary and voluntary member of a civilian militia that Chavez has built in recent years. "We're with him in the good times as well as the bad."
It was one of the largest rallies for Chavez in recent years. Public employees joined the president's supporters, and some arrived in government buses after traveling for hours across the country.
Chavez has undergone repeated surgeries, chemotherapy and radiation treatments since June 2011 for an unspecified type of pelvic cancer.
Information Minister Ernesto Villegas reiterated on Thursday that Chavez is "in a complex and difficult situation" due to a severe lung infection that led to a "respiratory deficiency." He didn't give details.
Many in the crowd said they were praying for the president.
Yet while Chavez's followers projected confidence in the resilience of their socialist movement, some also acknowledged the possibility of changes ahead.
"It's possible he may die," said Jaime Salcedo, a farmer who traveled across the country for the event. "But his death wouldn't be the end of the revolution. Of that I'm sure. Look at all those people in the street today."
Associated Press writers Fabiola Sanchez and Vivian Sequera contributed to this report.
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