CARACAS, Venezuela — Millions of Venezuelans signaled their disapproval of President Nicolás Maduro’s plan to hold a constituent assembly by casting ballots on Sunday in a vote unlike any other in this nation’s history.

More than 98 percent of voters sided with the opposition in answering three yes-or-no questions drafted with the aim of weakening Mr. Maduro’s legitimacy days before his constituent assembly is expected to convene. Opponents see the assembly as a power grab by an increasingly unpopular leader and fear he may use it to do away with democratic elections.

Sunday’s exercise, known as a popular consultation, was organized by a slate of opposition parties that dominate Venezuela’s National Assembly.

Organizers had hoped that a large turnout and a lopsided result would widen rifts within the governing party and deepen the government’s international isolation, undermining Mr. Maduro’s plan to appoint an assembly of handpicked supporters to draft a new Constitution.