(Reuters) - The mayor of Venezuela's capital Caracas will fight charges of plotting violence against the socialist government, his lawyer said on Saturday, denouncing the accusations as baseless.

Intelligence agents seized 59-year-old mayor Antonio Ledezma, 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 Venezuela's Ramo Verde military prison, where fellow hardline opposition leader Leopoldo Lopez has been jailed for a year.

The opposition said Ledezma's arrest was evidence that an increasingly unpopular Maduro was 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 sky-high crime.

But Ledezma's lawyer Omar Estacio says 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."

Officials have said a public document signed by Ledezma and two other opposition leaders urging a transition is a roadmap for an uprising. They said earlier this month he was among various politicians supporting a new plot with dissident military officers to topple the president via air strikes.

Ledezma is the highest-profile Maduro opponent arrested after Lopez, who was detained for his role in street protests that brought four months of violence last year and led to 43 deaths.

The mayor's arrest prompted isolated protests in the capital and fresh violence in the opposition stronghold of San Cristobal in western Venezuela, witnesses said.

"This is when people need to take to the street to defend democracy, without succumbing to violence, which would only benefit the government," Luis Pulido, a former member of Ledezma's media team, said on Friday at a small rally in Caracas.

While Ledezma's arrest has brought condemnation from the United States and several rights groups, it has so far failed to ignite major protests in the deeply polarized nation of 30 million.

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 oil-rich Venezuela.

(Editing by Stephen Powell)