Maduro's political opponents vowed earlier to step up protests over his plan to reform the constitution, which they say is a bid to cling to power.
Maduro has launched steps to set up a constituent assembly which the opposition says he plans to stack in his favor.
"If we allow the fraud that they want to call a constituent assembly, Venezuela will be lost," said Freddy Guevara, opposition deputy speaker of the National Assembly legislature.
He called on opposition supporters to "get ready for an escalation" of protests, but urged demonstrators to refrain from violence.
The opposition brand the leftist president a dictator and blame him for shortages of food and medicines. He accuses them of attempting a "coup" with US backing.
Prosecutors say 59 people have been killed in clashes since the protests erupted on April 1.
The government and opposition accuse each other of sending armed groups to sow violence in the protests.
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