Electoral authorities in Venezuela say the Socialist Party has won 17 of 23 state governorships in a crushing victory.
President Nicolas Maduro hailed it as a victory for chavismo, his party’s brand of socialism named after former president Hugo Chavez.
But opposition leaders alleged fraud.
The Democratic Union Roundtable coalition has refused to recognise the result and demanded a complete audit, campaign director Gerardo Blyde said.
Polls had suggested the opposition would win between 11 and 18 governorships.
Venezuela has faced serious food shortages, economic crisis and political violence this year, including the deaths of more than 100 people at protests.
Earlier this year the US placed sanctions on some Venezuelan officials. Treasury Secretary Steven Mnuchin said the country would not ignore Mr Maduro’s “ongoing efforts to undermine democracy, freedom, and the rule of law”.
Mr Maduro called the sanctions “illegal, insolent and unprecedented” and asked: “Who do these imperialists in the United States think they are?”
More conflict ahead
Analysis by the BBC’s Will Grant in Caracas
As the results came in, it was clear this would be a comprehensive victory for the government. One by one, the states were named and one by one they went to the candidates in red.
President Nicolas Maduro immediately appeared on state television, surrounded by his inner circle and members of the military. “Chavismo is back, victorious,” he told them, before urging the opposition to recognise the result too.
In fact, his opponents were already calling fraud. Mr Maduro’s government has only garnered around 20% support in recent polls and opposition supporters are saying the count can’t have been clean.
But as jubilant Maduro supporters dance in the plazas, this result may have repercussions outside Venezuela’s borders. The Trump administration is sure to side with the opposition and call this election fraudulent.
Just as the calm was beginning to return to Venezuela after months of violent street protests ended, this overwhelming government victory brings with it more uncertainty and the real possibility of further conflict ahead.