The government condemned the referendum as illegal, instead calling for a July 30 vote to elect a special assembly to rewrite the 1999 constitution.
Venezuela is struggling with a deepening economic crisis and sometimes bloody street protests as anti-government protesters demand Maduro step down. He has sent the military onto the streets to maintain order.
Tensions grew in late March when the Supreme Court transferred all National Assembly powers to the court
stacked with government loyalists. The opposition called the move a coup, and the court later revoked the ruling.
Ninety-nine people have died in the unrest, according to the Venezuelan attorney general's office. At least two people were killed during more protests Friday, including a 15-year-old.
Resignation in UN delegation
In a break with Maduro's government, a member of Venezuela's UN delegation -- Isaias Arturo Medina Mejías -- resigned Thursday, citing "irreconcilable differences" with his country's leadership.
Medina Mejías told CNN en Español he would join the resistance, returning to Venezuela as soon as the paperwork was in place.
He said he had been at odds with the government for months but had not resigned earlier because his passport had expired and he was waiting to renew it.
"I am in the UN, was working in the UN because I believed in what I was doing -- working with the international community representing my country," Medina Mejías said. "Today, I honestly believe that it is hypocrisy that Venezuela is even in the UN on the Human Rights Council, violating human rights."
His resignation drew condemnation from Venezuela's ambassador to the United Nations, Rafael Ramirez, who said in Spanish on Twitter
: "I condemn the conduct of Isaias Medina. We have immediately relieved him of his duties. He does not represent us. He has acted in a dishonest manner."
In a separate interview with CNN, Medina Mejías said the resignation was "an individual expression of my own willingness not to work with this government that for the last 100 days have violated civil rights, and has killed and systematically persecuted civilians and violated the constitution."
He said Venezuelans should "keep the pressure (up) until we get rid of this government and we can bring change to the self-determination of our country."
Venezuela's government intimidates and restricts the media in Venezuela, taking CNN en Español off the air.
The government tightly controls visas for foreign journalists including CNN, arresting those who report from inside the country without proper permits.
Fire and tear gas
Journalist Stefano Pozzebon described tense scenes Thursday in Caracas, with the national guard opening roads that young protesters would close minutes later. Tear gas was used in the clashes, he said.
In a nationally televised broadcast, Maduro accused "terrorists from the opposition" of setting a post office on fire in eastern Caracas used by the national police.
The mayor of the district of Sucre, where the blaze occurred, called for an investigation.
"Clashes between different groups of protesters was the source of fire in the police building in Los Ruices shopping mall. An action we denounce!" Mayor Carlos Ocariz said on Twitter.
He denounced the violence and said local police were immediately on the scene.
The 24-hour nationwide strike was called by National Assembly Vice President Freddy Guevara after Sunday's referendum in an effort to increase pressure on Maduro.
Nearly 7.2 million Venezuelans took part in the referendum organized by the main opposition parties, with the overwhelming majority coming out against Maduro's plan.
The referendum asked voters three yes-no questions. More than 98% of voters chose to reject the proposed constitutional assembly, request the military defend the existing constitution and support fresh elections before Maduro's term ends in 2019.
Sunday's turnout represented about 37% of Venezuela's total electorate, according to CNN calculations based on 2015 National Electoral Committee figures.
In 2013, Maduro was elected with just over 7.5 million votes, while Henrique Capriles was second with 7.3 million votes.
Maduro's supporters also cast a ballot Sunday in a rehearsal of the July 30 constitutional elections.
Competing lines could be seen throughout Caracas, with opposition and government-approved polling stations only meters apart. The opposition also set up 667 polling stations in 100 countries for expatriates.
Strains with Mexico
The Mexican Foreign Ministry on Thursday urged Maduro's government to halt the July 30 elections.
An official statement from Mexico said that suspending the vote would "open the door for a negotiation that could resolve the democratic crisis Venezuela is facing," whereas going forward with it would deepen divisions in the country.
Mexico's a "respectful and fraternal" request came days after Venezuelan Foreign Minister Samuel Moncada declared former Mexican President Vicente Fox had been banned from Venezuela for promoting violence and supporting foreign intervention.
Fox traveled to Venezuela on Saturday with a contingent of former Latin American presidents to show support for the referendum.
Fox told CNN that Maduro was imposing his will "with a stick in his hand -- with the guns and ammunition" and that he went to Caracas to be "by the side of these heroes, of these magnificent, young millennials, that are ready to die for their nation."
"Foreign support, foreign public opinion, leaders speaking about this massacre -- it will change things," Fox said.
US called 'xenophobic and racist empire'
Maduro's government also responded angrily to a US State Department statement
this week threatening "strong and swift economic actions" if Venezuela went ahead with a proposed special assembly to redo the constitution.
The United States encouraged other governments to call on Maduro to suspend the process and applauded "the courage and determination of Venezuelans who exercised their rights to freedom of expression, association, and peaceful assembly in defense of their democracy" in Sunday's vote.
Venezuela's foreign minister, Moncada, said the sanctions threat was "posed by a xenophobic and racist empire."
"The thin democratic veil of the Venezuelan opposition has fallen and reveals the brutal interventionist force of the US government, which has been behind the violence suffered by the Venezuelan people in the last four months," Moncada said at a press conference Tuesday.