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.