BOGOTÁ, Colombia — The government agents barged into the homes of two prominent former mayors, hauling them off to jail in the dark. They dragged one of them into the street in his blue pajamas as witnesses screamed that he was being kidnapped.

The two men, both vocal members of the opposition, had been arrested before. But as the doors shut and the cars sped away early on Tuesday, many Venezuelans worried that it marked the start of a new dictatorship in South America.

President Nicolás Maduro and his leftist movement have seized control of the country, not through a coup, but through a contentious power grab that has gutted Venezuela’s democratic institutions and effectively eliminated any official political challenges.

On Sunday, Mr. Maduro carried out an ambitious plan to consolidate power. He held a national vote, instructing Venezuelans to select from a list of trusted allies of the governing party — including his wife — who will rewrite the Constitution and rule Venezuela with virtually unlimited authority until they finish their work.