CARACAS, Venezuela — The bikers thundered up in a phalanx of red jackets and dark clothes, some with faces covered, revving motorcycles before a thousand protesters in Caracas. They threw tear gas canisters to disperse the crowd. Then, witnesses say, they pulled pistols and fired.

Someone fell. Carlos Moreno, 17, lay sprawled on the ground, a pool of blood around his head.

“His brain matter was coming out,” recalled Carlos Julio Rojas, a community leader who witnessed the fatal shooting in Venezuela’s capital on Wednesday.

The uniformed men who shot Mr. Moreno were not government security forces, witnesses say. Rather, they were members of armed bands who have become key enforcers for President Nicolás Maduro as he attempts to crush a growing protest movement against his rule.

The groups, called collectives or colectivos in Spanish, originated as pro-government community organizations that have long been a part of the landscape of leftist Venezuelan politics. Civilians with police training, colectivo members are armed by the government, say experts who have studied them.