"Venezuela today needs our staunchest support," Castro, 83, said in a rare international speech at a Group 77+China meeting in Bolivia yesterday.
"The oligarchs who could not get rid of President Hugo Chavez think the time has come to topple the Bolivarian revolution and President (Nicolas) Maduro," Castro argued, calling the elected socialist government in Caracas "the front line of independence, freedom and dignity."
Maduro is the closest regional ally of Cuba, the region's only one-party communist state.
Venezuelan economic support is critical to keeping the Cuban government and economy afloat.
But inflation near 60 per cent, widespread shortages of basic goods and soaring crime have plunged Venezuela -- an oil-rich OPEC member -- into political and economic crisis.
Anti-government protests have rattled Venezuela since February, leaving at least 41 people dead and more than 600 injured, with opposing sides trading blame for the violence.
G-77 host Bolivian President Evo Morales, also an elected socialist, said that if the United States meddles militarily in Venezuela, it will have a new Vietnam on its hands.
"Let us defend democracy, natural resources, our sovereignty and our dignity," he added.
Venezuela's government and the opposition held preparatory talks on April 7, which were followed by an unprecedented televised meeting between Maduro and opposition leaders, including the runner-up in last year's elections, Henrique Capriles.
Maduro, who succeeded late socialist firebrand Hugo Chavez, insists the meetings should not be negotiations. He has called for pacification and economic development programs.
