Venezuelan opposition leader Juan Guaidó has called for an uprising to oust President Nicolás Maduro.
In a video posted on social media, Mr Guaidó - who declared himself interim leader of the country in January - is backed by a number of men in military uniform.
He claimed the 'final phase' of the operation to take control of the country had begun.
The 35-year-old - flanked by another key opposition figure, Leopoldo López, who had been under house arrest - called on troops and members of the public to join an uprising.
En el marco de nuestra constitución. Y por el cese definitivo de la usurpación. https://t.co/3RD2bnQhxt
— Juan Guaidó (@jguaido) April 30, 2019
In a series of tweets, Mr Guaidó said: "The national armed forces have made the right decision - they have the support of the Venezuelan people, with the endorsement of our Constitution, with the guarantee of being on the right side of history.
"People of Venezuela, we need to go out on the streets together, to support the democratic forces and to regain our freedom."
Leading protests in the capital Caracas, he later insisted "the time is now" - calling on supporters to take to the streets.
However, Mr Maduro insisted military commanders were still on his side.
¡Nervios de Acero! He conversado con los Comandantes de todas las REDI y ZODI del País, quienes me han manifestado su total lealtad al Pueblo, a la Constitución y a la Patria. Llamo a la máxima movilización popular para asegurar la victoria de la Paz. ¡Venceremos!
— Nicolás Maduro (@NicolasMaduro) April 30, 2019
Photos and videos from the ground indicated there were some clashes between protesters and security forces, although the scale was not immediately apparent.
The country's Information Minister Jorge Rodriguez said a "small group of military traitors" were attempting a promote a coup.
He added that authorities were "confronting and deactivating" the group.
Informamos al pueblo de Venezuela que en estos momentos estamos enfrentando y desactivando a un reducido grupo de efectivos militares traidores que se posicionaron en el Distribuidor Altamira para promover un Golpe de Estado contra la Constitución y la paz de la República... 1/2
— Jorge Rodríguez (@jorgerpsuv) April 30, 2019
US Secretary of State Mike Pompeo reiterated the country's support for Mr Guaidó.
He said: "The US Government fully supports the Venezuelan people in their quest for freedom and democracy. Democracy cannot be defeated."
Meanwhile, Bolivian leader Evo Morales, who supports Maduro, condemned "the attempted coup d'état".
Crisis
Venezuela remains embroiled in a major political power struggle.
Concerns have long been raised about the human rights situation under Maduro's government - with reports of various abuses, including torture, by the country's security forces.
The country remains in the midst of a deep economic crisis, which has led to shortages of food and other vital supplies.
However, many commentators have also expressed unease about international efforts to support regime change in Venezuela, with many western countries recognising Juan Guaidó as the country's interim leader.
Critics have slammed such actions as a 'coup', and insist the Venezuelan people should be asked to choose their leaders through fresh and free elections without foreign interference.
The US has imposed crippling sanctions on the Maduro government, in particular targeting its oil supplies.
