Hundreds of people, many waving Venezuelan flags, flocked to a highway near a Caracas military base early Tuesday. Police responded with teargas as some of demonstrators broke away to throw rocks at security forces.
The government said it was "deactivating" an attempted coup by a small group of "treacherous" soldiers.
"We are currently facing and deactivating a small group of treacherous military personnel who took positions in the Altamira distributor road (in Caracas) to promote a coup d'etat," communications minister Jorge Rodriguez said on Twitter.
"We call on the people to remain on maximum alert to -- with our glorious National Bolivarian Armed Forces -- defeat the attempted coup and preserve peace," he said.
The US meanwhile threw its full support behind Guaido, with the White House calling on the military to protect the people and support the country's "legitimate institutions," including the opposition-controlled National Assembly.
"The US government fully supports the Venezuelan people in their quest for freedom and democracy. Democracy cannot be defeated," US Secretary of State Mike Pompeo said on Twitter.
In a video recorded at Caracas' La Carlota military air base and posted on social media, the US-backed Guaido said troops had heeded months of urging to join his campaign to oust Maduro.
"Today, brave soldiers, brave patriots, brave men supporting the constitution have answered our call," he said.
And Venezuela’s southern neighbour Brazil praised the move by Venezuelan troops to support Guaido and reiterated its hope for a democratic transition of power in its neighbour.
But the Russian Tass news agency largely ignored the reports, saying the Venezuelan armed forces remain loyal to the country’s “legitimate government.”
Russia, together with Belarus, Bolivia, Iran, China, Cuba, Nicaragua, El Salvador, Syria and Turkey support Maduro.
“All military units report that the situation in their quarters and at military bases under the command of their officers is under control," Tass quotes Venezuelan defence minister Vladimir Padrino Lopez as saying.