Following are some initial reactions from world governments:
The White House urged Venezuela's armed forces to stand by the country's "legitimate institutions."
President Donald Trump's administration has been waging a mounting pressure campaign to oust Maduro and install Guaido, who is recognized as acting president by more than 50 countries, including many in Latin America.
"The US Government fully supports the Venezuelan people in their quest for freedom and democracy. Democracy cannot be defeated," United States Secretary of State Mike Pompeo wrote on Twitter.
Today interim President Juan Guaido announced start of Operación Libertad. The U.S. Government fully supports the Venezuelan people in their quest for freedom and democracy. Democracy cannot be defeated. #EstamosUnidosVE — Secretary Pompeo (@SecPompeo) April 30, 2019
Colombia's President Ivan Duque called on Twitter for "soldiers and the people of Venezuela to place themselves on the right side of history, rejecting dictatorship and Maduro's usurpation."
Hacemos llamado a militares y al pueblo de #Venezuela para que se ubiquen del lado correcto de la historia, rechazando dictadura y usurpación de Maduro; uniéndose en búsqueda de libertad, democracia y reconstrucción institucional, en cabeza de @AsambleaVE y el Presidente @jguaido — Iván Duque (@IvanDuque) April 30, 2019
Colombia also said it was calling an emergency meeting of the Lima Group – a grouping of major Latin American nations plus Canada focused on Venezuela.
Bolivian President Evo Morales "vigorously" condemned the "attempted coup in Venezuela by the right-wing that is beholden to foreign interests."
He said he was sure that "the brave Bolivarian Revolution led by brother Nicolas Maduro will beat this new attack by the empire (the US)."
"We strongly hope that there will be no bloodshed," Spanish government spokeswoman Isabel Celaa told reporters.
She called for "democratic elections" and said Madrid supports a peaceful outcome to the Venezuela crisis.
Spain has thrown its support behind Guaido, whom it recognizes as interim leader of economically-strapped Venezuela over President Nicolas Maduro.
Celaa said Madrid thinks he "is legitimate to carry out the transformation of Venezuela."
But she added: "Spain is not supporting any military coup."
"We have been clear that the UK, alongside its international partners, recognizes Juan Guaido as the constitutional interim president of Venezuela until credible presidential elections can be held," a spokesman for British Prime Minister Theresa May said.
"Our focus is on a peaceful resolution to the crisis and the restoration of Venezuelan democracy. Venezuelan people deserve a better future, they have suffered enough and the Maduro regime must end."
"We reject this coup movement that aims to fuel violence in the country," Cuban President Miguel Diaz-Canel on Twitter.
"The traitors that have put themselves at the head of this subversive movement are using troops and soldiers with heavy weapons in a public area of the city to create panic and terror."
Rechazamos este movimiento golpista que pretende llenar de violencia al país. Los traidores que se han colocado al frente de este movimiento subversivo, han empleado tropas y policías con armas de guerra en una vía pública de la la ciudad para crear zozobra y terror #SomosCuba https://t.co/n1jIoABPvi — Miguel Díaz-Canel Bermúdez (@DiazCanelB) April 30, 2019
A European Commission spokeswoman said it was following developments but declined to comment on the fast-developing situation beyond repeating the bloc's long-standing call for "political and peaceful solution" and "free and fair elections."
However, European Parliament speaker Antonio Tajani hailed the release of leading Venezuelan opposition figure Leopoldo Lopez from house arrest as a "historic moment."
He tweeted that it was an important step in Venezuela's "return to democracy and freedom."
Lopez is a winner of the Sakharov Prize, an annual human rights award presented by the European Parliament. – Rappler.com