The assembly is expected to rewrite the constitution and give new powers to the ruling Socialist party.
Speaking with 15 other men dressed in camouflage standing by him, some of them armed, he demanded a transitional government and "free elections".
Also on Saturday, South American trade bloc Mercosur indefinitely suspended Venezuela, adding to worldwide pressure on Maduro to dismantle the newly created assembly and restore democracy.
Authorities said the men were mercenaries working for a US -backed opposition to bring down almost two decades of Socialism in oil-rich Venezuela, raising the specter of a further government crackdown on dissent in coming days.
On Sunday, the assembly announced the creation of a "truth commission" to probe alleged crimes committed during Venezuela's lengthy political crisis. Support for the president is diminishing amid a deep economic recession in the country, making room for opposition parties, which won the legislative election in 2015 and since then have the majority of the seats in the Venezuelan Congress.
"I denounce this arbitrary act before the national and worldwide community", she said.
In 2006, the United States prohibited the sale or transfer of military arms or technology to Venezuela, whose then-president, Hugo Chavez, had closely allied the country with Russian Federation and China.
But neither that nor condemnations and disapprovals are effective in a country where, while Venezuelans have been dying on street protests every day since February, their leader says the struggle is between the ninety-nine percent against the one percent.
UAE: Dubai Skyscraper Catches Fire Again
It marks the second fire at the skyscraper in the world-famous Marina district , with an eerily similar incident two years ago. Residents also received an emergency call from Torch Tower reception an hour after they had left to ensure they were safe.
More than ten countries of the region, among them Argentina, Colombia, and Peru, said that would not recognize the electoral results.
Maduro and his Socialist Party have "completely taken hostage" Venezuela's institutions through "an undemocratic mechanism that is utterly dictatorial", the leader of the opposition-controlled legislature, Julio Borges, told reporters.
In a sign of its cowed, demoralized state, only a few hundred demonstrators showed up for a Friday protest against the constitutional assembly, one of the smallest turnouts in months.
The opposition has vowed to maintain street protests against the assembly.
The South American has for months been in the throes of a political crisis with protests that have left more than 100 dead, almost 2,000 wounded and over 500 detained.
It might look like the United States has taken a step forward by deciding to impose sanctions on American firms and individuals doing business with Mr Maduro, but it is demoralising to learn that, on the other hand, Venezuela and the USA have been trading oil with each other.
"The suspension of Venezuela was applied due to the actions of the Nicolas Maduro government and is a call for the immediate start of a process of political transition and restoration of democratic order", according to a statement issued in Sao Paulo, where foreign ministers of the four countries met Saturday.