CARACAS (AFP) - Venezuela opposition leader Henrique Capriles announced on Wednesday he would formally contest results from last month's presidential poll, as both sides held rival May Day rallies in Caracas.

Mr Capriles told supporters at his demonstration in eastern Caracas that the formal challenge would be presented to the Supreme Court on Thursday. He had been under a Monday deadline if he wanted to file with the top court.

"We are going to exhaust all the internal fora because we have no doubt that this case is going to wind up before the international community," said Mr Capriles, who had challenged the late Hugo Chavez's hand-picked successor.

"This case is going to end up going to every country where there is a democracy." Tensions have soared since the April 14 vote, which official results gave to Nicolas Maduro, the leftist Chavez's chosen political heir, by a 1.5 per cent margin. Mr Capriles had lost to Mr Chavez himself last October by 11 points.

Mr Capriles, who has accused Mr Maduro of stealing the election, has rejected an audit of the vote by the National Electoral Council, begun on Monday, as a "farce," after it refused to look at physical voting records.

The council has limited itself to a narrow, technical review of the electronic voting system while insisting that no audit can reverse Mr Maduro's win.

Mr Maduro led a rival march in central and western Caracas - a last minute route change to avoid a confrontation with opposition marchers, who chose a major road in the east of the capital for their demonstration.

The president combatively insisted his "crybaby" rival should stop disputing the election outcome.

"You great big whopping fascist! You were defeated! Accept your loss already. Stop kicking and screaming and crying," Maduro told thousands of supporters in Plaza O'Leary, in a speech carried on national television.

"That's what you are: a little fascist bourgeois crybaby, who wants to take the country over a cliff sowing hatred and violence," Mr Maduro went on to say in a lengthy and bitter harangue.

The marches came a day after a brawl broke out in the National Assembly over the pro-Chavez majority's refusal to grant the opposition the right to speak until they recognized Maduro as the winner of the election.

The United States said it was "very concerned" about the development, with State Department spokesman Patrick Ventrell expressing "solidarity with those injured." "Let me speak clearly: violence has no place in a representative, democratic system and is particularly inappropriate within a National Assembly," he said.

Opposition politician Julio Borges said on Tuesday night he was beaten by ruling party members, and that he was "not the only one." Videos released by opposition deputies showed the lawmakers coming to blows.

Chavista deputy Elvis Amoroso defended his colleagues, saying if opposition lawmakers "do not recognise our constitutional president, then we do not need to recognize them, since they were also elected under the same voting system." At Maduro's rally on Wednesday, Zoraida Castro, 59, blamed the opposition.

"Due to their provocations (from the right), there is a tense atmosphere, because there is one sector that doesn't recognise Maduro and another that defends him," she told AFP.

Across town at Mr Capriles's rally, emotions were equally high.

"I'm in this march because of the discontent we are living in the country, for the dud elections. I feel my vote was stolen," said Ms Sara Clavico, 60, who works for the opposition-party mayor in the municipality of Chacao.

Hundreds of thousands of workers took to the streets of capital cities across Latin America to mark Wednesday's May Day celebrations. In Santiago, Chile, demonstrators clashed with police, leaving one policeman injured.