Protests erupted in Caracas late Friday after Venezuela's attorney general charged jailed opposition leader Leopoldo Lopez with responsibility for anti-government demonstrations that have shaken the country since February.
Riot police fired tear gas at crowds of rock-throwing demonstrators in the Chacao neighborhood in what has become an almost regular nightly ritual.
In an ominous sign of possible escalation, a masked man was photographed moving among the protesters armed with what appeared to be a rifle with a telescopic sight.
Leftist President Nicolas Maduro more than once has decried the existence of sharpshooters among the most radical protesters. The government claims the protests are part of a coup attempt orchestrated by the United States and right-wing Colombians.
The protesters, in turn, have long complained about harassment from gangs of armed, semi-official pro-government thugs.
Thirty-nine people have died and another 608 have been wounded since crowds took to the streets in early February to protest Venezuela's soaring crime, high inflation and shortages of essential goods.
Lopez, a Harvard-educated economist who is leader of the Popular Will party, has been held in a military prison since he was arrested on February 18 in the midst of a massive opposition protest rally.
Attorney General Luisa Ortega Diaz told a press conference that Lopez was formally charged with inciting violence, arson, damage to property and conspiracy.
The government had until Saturday to press charges or release him.
The action stems from Lopez's public support for the student-led street protests that have rocked the country since they began February 4 in the western city of San Cristobal and spread to several other major cities.
Lopez and some other opposition leaders support a strategy dubbed "the exit" that is aimed at generating pressure through street protests for Maduro -- the handpicked successor to late socialist icon Hugo Chavez -- to resign.
Hours after the attorney general's announcement, about 3,000 demonstrators, most of them students, hit the streets in eastern Caracas demanding that Lopez be freed.
Under the slogan "Free Leopoldo," some were dressed in white, a symbol of peace, while others carried the Venezuelan flag.
"We have no weapons, just stones," said one young man protester who wore a homemade gas mask.
Popular Will activist Ernesto Palacios, 31, told AFP: "As a sign of respect, the government should release Leopoldo Lopez and political prisoners."
From his cell at Ramo Verde military prison, Lopez, whose picture wearing a green shirt and leaning over a small window with hands on his cell bars has been circulated on social networks, has complained about his solitary confinement.
Lopez, who had called repeatedly for Maduro's resignation, is allowed one hour of exercise outdoors and has a small television.
Separately, Maduro announced the capture of two alleged drug traffickers he linked to the anti-government protests.
Colombian suspect Hugo Alberto Nuncira Soto, alias "Don Diego" and "El Junco," of the Los Urabenos criminal gang was arrested, Maduro said, blaming him for "guarimbas," or blocking streets with rubble.
Gabriel Alejandro Reyes Beltran of Venezuela, who is wanted by Interpol, was also arrested on charges of setting up barricades in San Cristobal.
"Criminal gangs of Colombian drug traffickers, paramilitaries and border security forces are directly involved in the planning and execution of roadblocks," the president complained on national media.
The president also ordered an investigation into armed vigilantes who entered the campus of the Central University of Venezuela in downtown Caracas, triggering violent clashes.