Al Jazeera's Lucia Newman, reporting from Caracas, said the defeat was a stinging blow to the government.
"This is even more than the opposition had hoped for. It's one of the strongest blows to the socialist revolution in the 17 years since it's been in power."
Fireworks erupted over the capital Caracas as opposition supporters celebrated.
Maduro's United Socialist Party of Venezuela won 46 seats in the single-chamber National Assembly, Lucena said. The results for 22 other seats had not yet been confirmed.
The result strengthens the opposition's hand against Maduro though it was short of the super majority the MUD had hoped for.
"Venezuela has won," tweeted senior opposition leader Henrique Capriles. "We always said this was the way: humility, maturity and serenity."