Venezuelan President Hugo Chavez won a majority in parliament but opponents took more than a third of the seats, threatening his ability to pass socialist reforms before seeking re-election in 2012.
Election authorities said that the ruling Socialist Party won at least 90 places, and the opposition Democratic Unity umbrella group at least 59, with all but 14 of the 165-member National Assembly seats decided.
A third party won another two.
The opposition said it had won 52 percent of the national popular vote in Sunday's election.
The results of the vote will buoy the opposition's hopes of toppling Chavez at the ballot box in two years' time.
The initial count was announced eight hours after the close of Sunday's voting because there were a number of close races, National Electoral Council president Tibisay Lucena said.
Ramon Guillermo Aveledo, leader of the opposition coalition, called the delayed results "inadmissible."
Before the vote, the opposition had criticized an election law passed by Chavez's allies that redrew some legislative districts and gave greater weight to votes in rural areas, where the president remains more popular. Opposition candidates agreed to participate in the elections and respect the results as long as the vote count was transparent.
The opposition, which boycotted the last legislative elections in 2005, dramatically increased its representation beyond the 11 or so lawmakers who defected from Chavez's camp in the current National Assembly.
Chavez's opponents achieved their goal of preventing his allies from obtaining at least a two-thirds majority of the 165 seats — a threshold at which pro-Chavez lawmakers have been able to rewrite laws unopposed and unilaterally appoint officials including Supreme Court justices and members of the electoral council.
A baseball-loving former tank soldier who rose from a poor rural childhood, Chavez first tried to take power in a 1992 coup and has only lost one election since he won the presidency at the ballot box in 1998.
He has become one of the world's most recognizable politicians, taking the crown from Cuba's Fidel Castro as Latin America's leading critic of Washington and putting swathes of Venezuela's economy in state hands.
Though not on the ballot for parliament, the 56-year-old Chavez was the dominant figure throughout the campaign, in yet another vote in Venezuela that was essentially a referendum on his rule.