Venezuelan Vice-President Nicolas Maduro said on Wednesday night that security services had uncovered a plot to assassinate him and parliament head Diosdado Cabello.
Mr Maduro revealed details of the plot at a rally commemorating the overthrow of former dictator Marcos Perez Jimenez, shortly before travelling to Havana to visit his cancer-stricken superior.
The acting president told supporters that "groups that have infiltrated the country" were planning to kill him or National Assembly president Mr Cabello and then blame the other for the attack.
The opposition has been keen to spread rumours of a split between the two key figures in the United Socialist Party, but the men have denied heading opposing factions and insist they are working well together.
The anti-Chavez camp held a smaller rally in a basketball arena to mark the January 23 1958 downfall of General Jimenez, accusing Mr Maduro of continuing Hugo Chavez's "dictatorship" in Venezuela.
But Chavistas were unconvinced.
"Our president has brought true democracy to our country," said bricklayer Angel Colmenares at the government rally in working-class Caracas neighbourhood 23 de Enero.
"Nobody should believe opposition leaders who say that Chavez is a threat to democracy."
Mr Chavez has not appeared publicly since going to Cuba for cancer surgery on December 10, but the government says his condition is improving.
Early editions of Spanish newspaper El Pais contained a front-page photo purported to be of Mr Chavez hooked up to breathing apparatus.
But they took it down after less than half an hour on their website, admitting that they had no proof it was him.