The Venezuelan government says it will hold elections for a controversial constituent assembly despite the threat of US sanctions.
The assembly would have the power to rewrite the constitution and to bypass the opposition-controlled legislature.
On Monday, US President Donald Trump said he would take “economic actions” if the constituent assembly went ahead.
Mr Trump also called President Maduro “a bad leader who dreams of becoming a dictator”.
Venezuelan Foreign Minister Samuel Moncada denounced Mr Trump’s words as an “insolent threat”.
Mr Trump had warned that “if the Maduro regime imposes its constituent assembly on July 30, the United States will take strong and swift economic actions”.
But he did not give any details as to what those actions may be.
Mr Trump’s warning came hours after the opposition said that 7.6 million people had voted against the constituent assembly in an unofficial referendum on Sunday.
The Venezuelan government disputed the figure given by the opposition and called it a “gigantic fraud”.
Calls within Venezuela and abroad have been mounting since the results of the referendum were announced.
Opposition politicians say Mr Maduro wants to use it to entrench himself in power, while Mr Maduro argues a new constitution will promote dialogue in the polarised country.
Colombia, France, Spain and the European Union have also demanded that the Venezuelan government drop its plan for the assembly.
European Union foreign policy chief Federica Mogherini warned it “risks further polarising the country and increasing confrontation”.
In response, Mr Maduro called her “insolent” and said that Venezuela was not a colony of the EU.
Venezuela also banned five former Latin American presidents from entering the country.
Venezuelan Foreign Minister Samuel Moncada said that Mexico’s Vicente Fox, Bolivia’s Jorge Quiroga, Colombia’s Andrés Pastrana and Costa Rica’s former presidents Laura Chinchilla and Miguel Ángel Rodríguez had been declared personae non gratae.
The former leaders had been part of a delegation who agreed to monitor Sunday’s unofficial referendum organised by the Venezuelan opposition.
Opposition leaders said they would step up their campaign against the constituent assembly.
They are planning a general strike for Thursday.