Opposition leaders called for more rallies on Thursday.

Carlos Moreno, 17, was fatally shot by a pro-government gang on Wednesday, according to witnesses and local news reports.

He was attacked after hundreds of pro-government gang members arrived and surrounded protesters, throwing tear gas canisters, Arturo Ríos, a witness, said in an interview. Mr. Ríos said the group then began to open fire.

“They shot the boy through the head,” Mr. Ríos said. “It’s not fair. He could have been a friend of mine.”

Provea, a human rights group, said a second person had been killed and 400 people had been arrested. On Wednesday night, the government said that a sniper had killed a member of the National Guard in an area outside Caracas.