Castro also asked the Organization of American States and its Secretary General Luis Almagro to "stop attacking Venezuela" and "manipulating reality".
The plebiscite will be held amidst the wave of anti-government protests that have been going on for over 100 days and have left 94 people dead.
Detractors say that move is aimed at giving Maduro dictatorial powers to hold on to office, notably by bypassing the opposition-controlled parliament.
Lawmakers approved a bill in December 2013 making Uruguay the first country to have a legal marijuana market covering the entire chain from plant to purchase.
"The opposition is boycotting the election and promoting an alternative round of balloting".
Ballots will ask voters if they support the government's plan to elect an unpopular National Constituent Assembly that would overhaul the 1999 constitution.
In the talks, the MUD demanded a timeline for elections, the liberation of "political prisoners", the distribution of much-needed food and medicine to the people, and respect for the autonomy of parliament.
Its campaign has found echo in the scarcities suffered by many Venezuelans. In Trinidad and Tobago, UNHCR is assisting the government to progressively assume responsibility for registration, issuance of documentation and refugee status determination.
Perez's stunt shocked many in Venezuela, although some suggest it was a false flag operation by the government to distract from the crisis.
Washington has sought to replace the Venezuelan government since 1998 when Hugo Chávez was elected president.
He urged the opposition to stop the "terrorist violence" designed "to oust" the president, and called for Maduro's opponents to accept the Bolivarian government's offer of dialogue.
But Maduro still enjoys loyalty from electoral and judicial institutions, and, most importantly, from his military chiefs.
"The tribute to the fallen ones is not just to be here in this moment". Opposition demonstrators with their faces covered routinely throw Molotov cocktails and fire homemade mortars at police and National Guard sent to break up the protests.
Granma, daily paper of the Communist Party of Cuba, featured an interview June 30 with Rogelio Enrique Suárez, a Cuban doctor who has been volunteering in Venezuela since 2014.
"Our concern regarding Venezuela is more present than ever", Guterres said in a statement.
Venezuela's neighbors have received more and more people who are in increasingly desperate situations.
Some of the major challenges posed by the large influx of Venezuelans in neighbouring countries include international protection and physical security considerations, lack of documentation, sexual and gender-based violence, exploitation and abuse, and lack of access to basic rights and services. The Cuban volunteers aren't afraid, but "we have to be aware of the dangers".
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