Although several opposition leaders had separately rejected Maduro's plan for a "people's" assembly, the center- right Democratic Unity Roundtable (MUD) formally announced yesterday that it would not participate.
"We cannot take part in a fraudulent process," former presidential candidate Henrique Capriles said, appearing at a news conference with other MUD leaders. "We have a constitution, and the government cannot repeal it by act of force."
But some analysts said they fear the opposition is leaving the way open for Maduro to change the constitution at will, possibly cementing his grip on power.
Opponents plan to march to the headquarters of the education ministry in central Caracas instead to explain their position.
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MUD rejects Maduro's proposal to elect half the members of the constituent assembly by voting in sectors it says are government-controlled, while the rest would be chosen in municipal elections.
Capriles maintains that Maduro -- whose leadership is rejected by seven of 10 Venezuelans, according to polls -- is trying to avoid a general election, the main demand of the opposition protests since April 1.
Opposition leaders said yesterday that they plan an intense day of protests to demand elections and an end to the "repression" of demonstrations, which have left 36 people dead and hundreds injured or jailed.
Citizen assemblies would be held Tuesday to explain the "flaws" in the constituent assembly, deputy Stalin Gonzalez said.
He said there would be a march Wednesday on the Supreme Court and other judicial offices around the country, and other protests Thursday and Friday.
None of the demonstrations has managed to reach the center of the capital, considered a bastion of Chavismo, the left-wing ideology created by Maduro's predecessor, Hugo Chavez.