Caracas (AFP) - Venezuela's opposition called a nationwide strike for Thursday to press President Nicolas Maduro to back off a rewriting of the constitution, ratcheting up tensions after an unofficial vote rejecting Maduro's plan and amid months of deadly protests.

The strike call, issued on Monday, was part of what the opposition called a "final offensive" aimed at forcing Maduro out through early elections before his term ends in 2019.

On Sunday, in an event organized by the opposition, more than a third of Venezuela's 19 million voters rejected Maduro's bid to have a citizens' body called a "Constituent Assembly" elected on July 30 to redraft the constitution.

Several countries lauded the balloting. White House spokesman Sean Spicer said Monday it sent an "unmistakable statement" to Maduro and his government.

The EU's top diplomat, Federica Mogherini, said that Maduro should suspend his plan, or he "risks further polarizing the country and increasing confrontation."

However Maduro and his government, backed by a loyal military, have dug in against the opposition tactics and the international criticism.

Despite growing public anger at food and medicine shortages under a spiralling economic crisis that has fed into the opposition movement, authorities in Caracas portray the efforts against them as illegitimate and the result of interference from the "imperialist" United States.

- 'Escalation' to follow -

"We are calling all the country to take part in a massive and violence-free protest through a nationwide civic strike for 24 hours," said one leader in the opposition coalition, Freddy Guevara.

He said the stoppage was a "mechanism for pressure and to prepare for the definitive escalation to take place next week."

There were fears, however, that the stepped-up confrontation could worsen violence in Venezuela's streets. Since April, when anti-Maduro protests and police pushback turned bloody, 96 people have died.

The opposition set the scene for the strike with its vote Sunday, which it called a "plebiscite" but which the government dismissed as "illegal."

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Electoral authorities, who have systematically sided with Maduro against the opposition-controlled legislature, denied authorization for the balloting.

A total of 7.6 million Venezuelans -- at home and abroad -- turned out for Sunday's vote, the opposition said, undermining legitimacy for Maduro's future Constituent Assembly.

Brazil's foreign ministry said in a statement "the high turnout in the plebiscite... was an unmistakable sign the Venezuelan people want democracy quickly restored." It, too, called on Maduro to shelve his Constituent Assembly idea.

- Change wanted -

Venezuela's opposition, invigorated by the voter support and the international reactions, clearly was keen to seize the moment.

"The mandate the people have given us is clear," said Julio Borges, leader of the opposition-controlled National Assembly.

Borges said the vote showed a public desire to see Maduro leave power before his term ends.

Political analyst John Magdaleno told AFP that "there is evidence of a persistent and durable demand for political change."

The result of Sunday's vote may not have been binding, but Venezuela "sent a clear message to the national executive and the world," announced Central University of Venezuela president Cecilia Garcia Arocha, one of several experts who oversaw Sunday's vote.

According to the opposition, the final turnout figure was enough to overturn Maduro's mandate should there be a recall referendum, because it exceeded the 7.5 million votes that put the president in power in 2013.

To lend weight to the vote, a group of former Latin American presidents, including Mexico's Vicente Fox, who was declared "persona non grata" by the government, took part as observers.

But Luis Vicente Leon, head of the polling firm Datanalisis, said the opposition's challenge now was to leverage the vote to "crack" Maduro's stance and "press for negotiations that would give an peaceful chance for change."

The opposition has accused Maduro of driving the country into bankruptcy, and of planning to use the Constituent Assembly to entirely sideline the legislature.

For many ordinary Venezuelans suffering under shortages of basic goods, sky-high inflation and climbing unemployment, the vote was a way of expressing frustration at the president and his policies.

Yet Maduro has insisted his proposed Constituent Assembly is "the only path" to peace and economic recovery. Thus far, he has shown no sign of backing down.