Several opposition leaders in Venezuela have put forward claims that they have won a majority of seats in elections for the National Assembly.

However, several hours after polls closed there are no official results.

Earlier, squabbles broke out at some polling stations with members of the opposition complaining of foul play after the electoral council extended voting by an extra hour.

The governing socialists face their first major challenge in 17 years.

"The results are as we hoped. Venezuela has won. It's irreversible," tweeted Henrique Capriles, a leading opposition figure and former presidential challenger.

But there was no indication of the basis on which he and other opposition leaders were making their claims.

Opinion polls before Sunday's vote suggested a broad opposition coalition could capitalise on widespread frustration over food shortages, inflation and crime.

The governing United Socialist Party of Venezuela (PSUV), however, retains wide support in rural areas, and will continue to control the presidency.

All 167 National Assembly seats were being contested.

The BBC's Candace Piette says that even if the opposition Democratic Unity Roundtable captures a majority in the assembly, its power will be limited.

However, a win for the coalition - which includes centre-left and centre-right groups - could mark a potential political shift in the country, our correspondent adds.

Analysis: Wyre Davies, BBC News, Caracas

Just in case they had missed all the pre-election publicity and coverage, residents in the capital were woken at 05:00 by a series of alarms and sirens to remind them of their civic duty.

Voting was brisk, both in areas regarded as pro-government and in others leaning towards the opposition. Many voters who came out early proudly held up their little fingers, stained with purple dye, to show they had already voted.

Although the voting process itself, which is largely electronic, is widely regarded to be clean, the ruling socialist party, in particular, has repeatedly been accused of bending election rules. There was some evidence of that in Caracas, with pro-government posters and some supporters outside a polling station in Bellas Artes. This is not usually allowed. But there has been no sign of intimidation or fraud and, thus far, the national election council says things are running smoothly and correctly.

Venezuelans are well used to queuing for hours in long lines for basic goods as the economic crisis deepens. Waiting patiently to vote in these nationwide Congressional elections is one thing they don't mind queuing for.

The elections were widely seen as a referendum on President Nicolas Maduro, the handpicked successor of the late president Hugo Chavez, and the party's socialist policies.

The opposition accuse the PSUV of mismanaging the economy and of squandering the country's oil wealth.

Mr Maduro says his party defends the interests of ordinary Venezuelans and wants to complete Mr Chavez's "Bolivarian Revolution".

Among the campaign issues were chronic food shortages of staples - such as milk, rice, coffee, sugar, corn flour and cooking oil.

Mr Maduro has blamed the situation on an "economic war" waged against his government by the opposition.

The opposition also accuses the government of increasing authoritarianism.

Earlier this year opposition leader Leopoldo Lopez was given a 13-year prison sentence for inciting violence - a charge critics say was politically motivated.

Venezuela invited election monitors from regional body Unasur but rejected those from the Organisation of American States (OAS) and the European Union.