"The call we are now making to our people is to stay in the streets in this quest of freedom and for our armed forces to continue to join," Carlos Vecchio, whom the United States recognizes as Venezuela's ambassador, told reporters in Washington.
He renewed calls for Maduro, who has been declared illegitimate by most Western and Latin American nations but is allied with Russia and China, to cede control.
"This is the time to leave power. It's time for your ambition to stop causing Venezuela more suffering," Vecchio said.
Guaido, the leader of the opposition, had called on Venezuelans to rise up on the May 1 workers' holiday for what he vowed to make the biggest rallies in the country's history.
Guaido said Tuesday that troops had joined him, although Maduro insisted he maintained the support of the top military leadership and condemned what he called a coup attempt.
Vecchio rejected his assertions, saying, "The coup d'etat is by dictator Nicolas Maduro, who is usurping power."