CARACAS, Venezuela — The agents barged into the home of Yonaiker Ordóñez, 18, on Sunday morning as he slept. Dressed in helmets and carrying rifles, the men grabbed the teenager and forced him to another room without explaining why they came, his family said.

“They took him to the area behind and killed him there,” said his sister, Yengly González.

The operation resembled one of the many police raids against the gangs that terrorize Venezuela’s poor neighborhoods. But Mr. Ordóñez’s only crime, his family said, was that he attended a protest against the government days before.

President Nicolás Maduro is facing the biggest challenge to his authoritarian rule yet. Protesters are in the streets, an opposition lawmaker has declared himself the rightful president, a growing number of foreign governments have backed that claim and the Trump administration has intensified the pressure, cutting off Mr. Maduro’s access to oil sales in the United States — a principal source of his government’s cash.

In the face of the crisis, Mr. Maduro has hit back hard, sending out security forces to crush dissent in deadly operations that have alarmed even some of the president’s traditional supporters.

For months, he continued to attack military bases and taunt the government on social media.

In an interview with The New York Times shortly before his death, Mr. Pérez asserted that a pro-Maduro paramilitary group had penetrated the special police unit and exerted influence over it. It was an explosive assertion even then, because it meant that civilian vigilantes were acting as uniformed police officers.

The day of Mr. Pérez’s death, the leader of the paramilitary group, a man named Heiker Vásquez, was killed fighting alongside FAES officials who had surrounded Mr. Pérez.

Uniformed members of the special police unit were also photographed in Mr. Vásquez’s funeral procession along with members of his paramilitary group, known as the Three Roots. In Venezuela, these armed paramilitary groups are known as “colectivos,” and typically have their roots in fervently pro-Chávez circles.

“If it’s not FAES in these raids, it’s colectivos dressed in FAES uniforms,” said Ms. Solórzano, the legislator, saying that she believes the pro-government groups are being armed and asked to fight alongside regular officers.

Julio Reyes, an opposition activist, said he was targeted on Sunday in the Tacagua Vieja neighborhood by the special police unit.

Just after he had gotten up and his wife began making coffee, he said he heard motorcycles revving in front of his house. Six masked men barged into his home, forced his family onto a couch and pointed a gun at his wife and him, he said. After a short period, they left.