Venezuela's parliament failed to debate new legislation making it easier for the government to pass security and media laws when members loyal to President Hugo Chavez stayed away.
The opposition has scented victory - in parliament at least
The pro-government MPs accused the opposition of planning to use violence against them in the chamber while the opposition said Mr Chavez's supporters merely feared they would not win a majority.
Parliamentary business has been in turmoil since last Friday when pro-government MPs held a bizarre open-air, alternative "session" in the capital, Caracas, after scuffles a few days earlier in the chamber.
The dispute is threatening to disrupt opposition efforts to have a mid-term referendum held on the maverick president's rule and the boycott comes on the eve of the latest planned mass protest against him.
"We won't present ourselves in the chamber while this climate of violence against us persists," said Nicolas Maduro, an MP from Mr Chavez's Fifth Republic Movement.
Chavez recently survived a coup, then a crippling strike
The opposition has threatened to prevent pro-government members from physically taking their seats in the chamber in protest at their insistence that Friday's open-air session be declared valid.
The opposition narrowly won a vote on the issue on Tuesday - a rare defeat for President Chavez's supporters - but the pro-government members said they would press for a new one. The opposition promptly declared this illegal.
One opposition MP, Rafael Octavio Rivero, said the government boycott was proof that it was "not capable of facing defeat because they
simply are not democrats".
The dispute comes a day before an anti-Chavez demonstration which the opposition plans to hold in the city's eastern Petare district, considered a government stronghold.
A similar protest in May in another pro-Chavez district of the city, Caricuao, led to violent clashes in which one person was killed.
Chavez supporters say they face violent attack in the chamber
The opposition has long been pushing to oust President Chavez whose radical, leftist policies and populist style of leadership have alienated the Venezuelan establishment as well as many liberals.
Following the defeat of a general strike which damaged the vital oil industry earlier this year, the opposition has focussed on having a referendum held on the elected president's rule: a move which the government appeared to accept in a deal brokered by the Organisation of American States in May.
But the referendum has to be organised by a new National Electoral Council, which cannot be appointed without a parliamentary vote.
The government has a wafer-thin majority in the unicameral 165-seat National Assembly but on Tuesday it fell one vote short of the 83 votes needed to pass its motion after three MPs abstained and a fourth failed to turn up.