Lawyers for Antonio Ledezma, the jailed opposition mayor of Caracas, say they will ask a judge to dismiss conspiracy charges against him, calling accusations that he participated in a plot to overthrow Venezuela's Socialist government "totally unfounded".
Omar Estacio, Ledezma's lawyer, said on Saturday he believes in Venezuela's justice system and he planned to lodge a first appeal as early as Monday or Tuesday.
"We're going to appeal the judge's decision," he said.
"I feel very confident that the Venezuelan justice will rectify this because these charges are truly unfounded."
Intelligence agents seized Ledezma, 59, a trained lawyer, at his office on Thursday night.
He was indicted the next day on conspiracy charges against President Nicolas Maduro and is held at the Ramo Verde military prison, where Leopoldo Lopez, a fellow opposition leader, has been jailed for a year.
Ledezma's arrest has prompted isolated demonstrations in Caracas and fresh violence in the opposition stronghold of San Cristobal in western Venezuela, witnesses say.
The government's case against Ledezma appeared to arise from a public letter he wrote with two other opposition leaders calling for a transitional government.
Maduro said the letter, published in an anti-government newspaper, was the green light for a secretly hatched putsch with US embassy involvement.
The US called the accusations "baseless and false".
Government officials also said Ledezma was among various politicians supporting a new plot with dissident military officers to topple Maduro via air strikes.
Ledezma is the most prominent Maduro opponent arrested after Lopez, who was detained for his role in last year's protests that brought four months of violence and led to 43 deaths.
Ledezma's daughter Antonietta urged Venezuelans on Saturday to take to the streets amid what she termed a "critical moment" for the country.
Separately, Uruguay, which currently holds the presidency of South American bloc Unasur, said it was coordinating an imminent trip by foreign ministers of Brazil, Colombia and Ecuador to meet Maduro in Caracas.
While Ledezma's arrest brought condemnation from the US and rights groups, it has so far failed to ignite major protests in deeply polarised Venezuela.
Many Maduro supporters loathe Ledezma, whom they call "the Vampire", and say he is part of an undemocratic, elitist clan intent on recouping power in Venezuela.
The arrest of Ledezma comes as the government struggles to avert a crisis made worse by a recent fall in oil prices.
The opposition says Ledezma's arrest indicates an increasingly unpopular Maduro is cracking down on dissent to hold on to power and distract citizens from chronic shortages of basic goods, the region's highest inflation, a recession and rampant crime.
Maduro's approval rating was hovering around 22 percent in January, the lowest in 16 years of Socialist rule.
Maduro has taken to the airwaves to rail against his opponents, accusing them of conspiring with the US to sabotage the oil-dependent economy and carry out a coup.