Defense Minister Vladimir Padrino, a general, accused Guaido of attempting a "coup d'etat" and said Maduro, 56, is "the legitimate president."
Eight generals who command of strategic regions of the country reiterated their "absolute loyalty and subordination" to the socialist leader in messages carried on state television.
Some ended their statements by adding: "Always loyal, never traitors." In Washington, US Secretary of State Mike Pompeo warned the Maduro government against the use of force against demonstrators in a speech to the Organization of American States.
"The time for debate is done. The regime of former president Nicolas Maduro is illegitimate," Pompeo said. Guaido, 35, set the showdown in motion Wednesday by proclaiming himself "acting president" and was swiftly recognized by the United States and about a dozen regional governments, including those of Brazil, Argentina and Colombia. Mexico, Cuba and Bolivia, all in the hands of leftist governments, pledged support for Maduro. Further afield, Maduro received support from allies Russia and China while France and Canada backed Guaido.