Embattled socialist President Nicolas Maduro, who lost control of Venezuela's National Assembly in polls earlier this month, said on Tuesday that fraud had marred the vote, which was being investigated.

"I am not going to release preliminary results of the investigation ... There were a lot of things going on. Buying people's votes, ... control of members of polling station teams and entire polling station teams," Maduro said on his weekly television show.

Venezuelan opposition leaders Tuesday claimed President Nicolas Maduro's leftist party had filed a legal challenge against the election of 22 opposition lawmakers in this month's landmark polls.

The opposition coalition MUD won a resounding victory in the December 6 legislative vote, in a shift that would end 16 years of leftist majority in the oil-rich but troubled nation.

The election was three days ago, but near the crisp white facade of Venezuela's presidential palace hundreds of noisy voters are still in campaign mode.

"Struggle! Battle! Victory, my friends!" yells a man with a loudspeaker.

President Nicolas Maduro has shifted into damage control mode, asking his cabinet to resign in oil-rich but troubled Venezuela after the opposition won a majority in the legislature in a landmark challenge to his grip on power.

"I have asked the council of ministers to present their resignation in order to carry out a process of restructuring, renovation and deep reform in the government," Maduro said on TV late Tuesday.

Recent electoral upsets in Argentina and Venezuela could herald a crumbling of the leftist bloc that has prevailed in Latin America over much of the past two decades, analysts say.

Tumbling commodity prices have thrown the region into economic turbulence, while many voters are said to be tiring of their leftist leaders -- though those in countries such as Ecuador and Bolivia seem to be going strong.

The defeat of Venezuela's ruling Socialists in legislative elections on the weekend was but a temporary setback, a key leftist ally in Latin America, Nicaraguan President Daniel Ortega, said Monday.

"As difficult as the horizon seems, we have no doubt: we will live on, and we will win," Ortega said in a public letter sent to his Venezuelan counterpart Nicolas Maduro.

Venezuela's opposition won control of congress for the first time in 16 years Monday as voters punished the socialist government for an economic crisis and insecurity in the oil-rich nation.

President Nicolas Maduro promptly accepted the defeat, a blow to his leadership and the "revolution" of "21st century socialism" launched by his late predecessor Hugo Chavez.

Two huge black eyes stare out from the top of the tower block housing the National Assembly in Caracas, surveying the busy streets and the misty green mountains beyond.

Just the eyes -- but every Venezuelan knows whose.

Venezuela's opposition will fight to seize control of the legislature from the socialist government in elections on Sunday that leaders warn risk sparking violence in the crisis-hit major oil producer.

For the first time in 16 years of "Bolivarian revolution" under late president Hugo Chavez and his successor Nicolas Maduro, polls show their rivals could now win a majority in the National Assembly.

Its leaders have been thrown in jail, banned from politics and remain deeply divided, but Venezuela's opposition nonetheless looks set to bruise President Nicolas Maduro by winning control of the National Assembly.

Sixteen years into late president Hugo Chavez's leftist "revolution," opinion polls indicate the opposition is poised to win legislative elections Sunday for the first time since the firebrand leader came to power.