Venezuela's opposition officials called for a nationwide strike against President Nicolas Maduro to protest against his plan to rewrite the constitution, ratcheting up tensions after they held an unofficial vote rejecting his proposal.

The 24-hour strike planned for Thursday, 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.

"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.

OPINION: Maduro is not Chavez

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

Maduro, meanwhile, called for national dialogue.

"I am willing to speak, to dialogue always," he said. "Time is running ... I am calling them to dialogue for months, but the divisions between [the opposition] do not leave them."

Al Jazeera's John Holman, reporting from Cucuta in neighbouring Colombia, said that the opposition was "stepping things up".

"Basically it is all or nothing for them," he said.

World reacts

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

US President Donald Trump weighed in on Monday, warning Caracas of "swift economic actions" if Maduro delivers on his bid, calling the Venezuelan president a "bad leader who dreams of being a dictator".

"The United States will not stand by as Venezuela crumbles," Trump said in a statement, as he praised the opposition's unofficial poll.

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

Maduro responded to Mogherini in a television broadcast.

"[Mogherini] tried to give orders to the government of Venezuela. 'Insolent', she believes that we are in 1809 when we received orders from European empires," he said. "Venezuela is a free and sovereign country and no one gives orders ... You chose the wrong country. Venezuela is not a colony of the European Union, it is not a colony of Europe."

Meanwhile, Brazil's foreign ministry praised the high turnout in the plebiscite and called on Maduro to shelve his Constituent Assembly idea.

Colombian President Juan Manuel Santos plans to discuss the crisis during a visit with President Raul Castro of Cuba, Venezuela's closest regional ally, Colombia's foreign minister said.

'Zero hour'

Maria Corina Machado, leader of the Vente Venezuela (Come Venezuela) movement which is part of the opposition coalition, said that the opposition has begun a final "zero hour" phase.

In this phase, she said, the government must abide by the mandates of the majority of Venezuelans, which include accepting the referendum.

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

Despite growing public anger over a spiralling economic crisis, authorities in Caracas portray the efforts against them as illegitimate and the result of interference from the "imperialist" United States.

There are 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 on Sunday, which it called a "plebiscite" but which the government dismissed as "illegal."

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

The president of the National Electoral Council (CNE) of Venezuela, Tibisay Lucena, said that the referendum convened by the opposition has no validity and asked the leaders of the opposition sector not to generate "false expectations" in their militancy.

Academics who oversaw the symbolic poll as guarantors of its credibility counted a turnout of more than seven million voters, undermining legitimacy for Maduro's future Constituent Assembly.

Political analyst John Magdaleno told AFP new agency 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.

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.

Maduro and the military dominate most state institutions but the opposition controls the National Assembly and holds three of 23 governorships. The country's chief prosecutor has recently broken with the ruling party.