Venezuelan National Assembly President Julio Borges said via Twitter that more than 7.2 million people took part in the vote, calling the turnout a clear rejection of Maduro.
The referendum comes almost two weeks before a Maduro-backed vote for a constituent assembly that would be tasked with rewriting the constitution without the opposition controlled national assembly.
Julio Borges, head of the opposition-run National Assembly, said that he hopes the popular consultation will serve as "a great quake that shakes the conscience of those in power".
Venezuelans living overseas were also given a chance to vote, with impromptu stations in the US, Italy, Spain, Mexico and Columbia - all popular destinations for the ever increasing number of refugees fleeing the crisis in Venezuela.
Still, Maduro has vowed that Sunday's referendum will do nothing to stall a July 30 election for delegates for a constituent assembly, which will be responsible for rewriting the country's 18-year-old constitution.
Speaking at an afternoon news conference, opposition leader Freddy Guevara made little mention of more protests, however, a sign that the opposition had chose to change tactics in the wake of Sunday's vote.
The vote in the unofficial, Opposition-organised referendum was just short of the 7.7 million who voted for the Opposition in Congress.
Monitors of the referendum say 7.2 million people cast ballots, in a country of around 19 million registered voters.More news: Jordan imprisons soldier who killed USA military trainers
Recounting the votes at one of the polling stations in Caracas on Sunday July 16, 2017. Maduro's allies accused the opposition of inflating numbers with multiple voting and false registrations.
Over the past couple of years the escalating crises in what once was the richest South American country has led to steep deterioration of public health with infectious diseases such as malaria creeping back.
The Brazilian government says an opposition-organized referendum in Venezuela is "unequivocal proof" that the people there want to see the restoration of the democratic rule of law.
"Tomorrow in the National Assembly convening [.] will receive the final results (of Sunday's plebiscite) and will give the commission the ability to name the new magistrates, a process that will culminate on Friday with the naming of the new magistrate in the Supreme Court of Justice of Venezuela", Guevara.
The referendum Sunday was also marred by violence.
Opposition strategy may include lengthy road blockades and sit-ins, a national strike, or possibly a march on the Miraflores presidential palace, similar to events before a short-lived coup against Maduro's predecessor Hugo Chavez in 2002.
Foreign Minister Samuel Moncada said some media passed off photos of government supporters participating in the practice for July 30 as if they were opposition sympathizers at Sunday's referendum. The opposition has called for a boycott of that vote. Still, some supporters said they were disappointed. The coalition has said it will do that in coming days to keep the vote secret and prevent reprisals by officials. Opposition leaders said that was because they were only able to set up 2,000 polling places in a symbolic exercise the government labeled as illegitimate.
It is unclear what changes Maduro plans to enact, but opposition members fear any branch of government that doesn't fall in line with Maduro will be left powerless.