Venezuelan Vice President Nicolas Maduro said unidentified groups had entered the country with the aim of assassinating him and the head of the National Assembly as President Hugo Chavez recovers from cancer in Cuba.
Maduro provided no proof of the, made at a rally on Wednesday to mark the end of a dictatorship in the OPEC nation 55 years ago, but he said action would be taken shortly.
"For several weeks we've been following groups that have infiltrated the country with the aim of making attempts on the life of (Assembly head) Diosdado Cabello and my own," Maduro told a crowd of red-shirted "Chavista" supporters. "They will not manage it against either of us."
Chavez named Maduro as his preferred successor before he went to Cuba in early December for surgery, his fourth operation in 18 months for an undisclosed form of cancer in his pelvis that was first diagnosed in mid-2011.
Chavez has not been seen in public nor heard from since then. Venezuela's government says his condition is improving after he suffered multiple complications caused by the December 11 surgery.
Officials say he is in "good spirits" but no date has been set for his return home. Maduro said he and the energy minister would travel to Havana on Wednesday to see Chavez.
Uncertainty over the 58-year-old president's fragile health has raised the specter of political instability in the deeply polarized South American country of 29 million people.
During his 14 years in power, Chavez has repeatedly accused Venezuela's "traitorous" opposition leaders of plotting to kill him, but offered little proof.
The opposition says the charges are a smokescreen to distract from Venezuela's daily problems such a shortages of staple goods, high inflation and one of the worst crime rates in the world.
"Now Maduro comes with the little story that we want to see an attempt against his life and that of Al Capone," opposition leader Henrique Capriles said on, referring to Cabello as the Prohibition-era U.S. gangster. "Absolute nonsense!"
Both the opposition and the ruling Socialist Party had originally planned large marches for Wednesday January 23.
It is an emotive date for Venezuelans that recalls the day in 1958 when military dictator Marcos Perez Jimenez fled the country amid widespread riots and a coup by rebel soldiers.
That had raised the possibility of supporters clashing in the streets, but the opposition subsequently decided to hold a meeting instead at a large hall in eastern Caracas.
In the city center, three separate marches by Chavez supporters converged on the 23 de Enero neighborhood, which was named after the day Jimenez was toppled from power. The demonstrations were smaller than other protests held in recent months.
Wearing a red and white tracksuit and speaking from the stage, former bus driver Maduro said the words the opposition used about him and Cabello - an ex-army buddy of Chavez - betrayed their disdain for Venezuela's poor.
"It is the condescending language of an oligarchy that will never understand who we are, who Chavez is, who the people are," Maduro told the crowd, many of whom waved posters of Chavez or wore T-shirts emblazoned with the image of his eyes.
"They say 'We must to get rid of that little lieutenant and that bus driver,'" Maduro said. "Don't be surprised by the actions that are taken in the coming hours and the coming days. The criminals who infiltrate our country can't ask for mercy."