'We do not recognize any of the results at this time. We are facing a very serious moment for the country,' said Gerardo Blyde, the Democratic Union Roundtable's (in Spanish Mesa de la Unidad Democratica, MUD) campaign director, who called for a full audit of the results.
Polls had projected MUD coalition candidates were likely to win over half the contests.
Late Sunday, however, the pro-government president of the electoral commission Tibisay Lucena said the ruling Socialist Party of President Nicolas Maduro won 17 governorships, versus five for the opposition MUD coalition.
Maduro, 54, hailed the victory as a win for "Chavismo," referring to the ruling movement's name for former president Hugo Chavez, and for democracy.
'Today Chavism devastated (the opposition). Today we have 17 governorships. Today we have 54% of the votes. Today we have 61 percent participation. And today the homeland has strengthened with 75% of the governorships (determined),' Maduro said in an exuberant, televised speech.
Minutes earlier, opposition leaders had warned the pro-government election commission was about to announce results that contradicted their certainty of what they described as a 'gigantic' victory. 'We have serious suspicions and doubts,' Blyde told reporters.
According to official results, the opposition's five wins included the restive Andean states of Merida and Tachira, and the oil-producing region of Zulia, but lost the populous Miranda state, which includes part of capital Caracas.
The poll was seen as a test for the Maduro government in the face of rising discontent in Venuezuela over food shortages, a ruined economy, and rampant crime. The vote and the government's handling of it is being closely watched by the United States, which on Friday called for the Venezuelan government to allow for a free and fair election.
U.S. State Department spokeswoman Heather Nauert on Friday expressed Washington's concern over the Maduro government's refusal to permit independent electoral observers. 'The United States is concerned that a series of actions by the National Electoral Council calls into question the fairness of the electoral process,' she said.
Many polling places stayed open past the official 6:00 pm closing time to allow voters already in line to cast their ballots.
Official sources had estimated the turnout at around 60 percent.
Even if opposition politicians had won a majority of state governorships, they still would have not been able to effectively wield power, because Maduro had said incoming governors would have had to take an oath of office and 'subordinate themselves' to the Constituent Assembly.
Maduro created the pro-government legislative body last summer following months of protests. The assembly seized legislative power from the opposition-dominated congress.
The opposition hoped the poll would signal the start of the end of two decades of socialist rule. Venezuela has been rocked by protests that have left more than 100 dead since April. The unrest has been fueled by persistent food shortages, rising political repression, and crime.