The US has branded Venezuelan President Nicolas Maduro a "dictator" threatening his country's democracy and has imposed sanctions targeting any assets he might hold on US territory.
It comes in response to Sunday's Venezuelan election that Washington has denounced as a "sham" vote.
"Yesterday's illegitimate elections confirm that Maduro is a dictator who disregards the will of the Venezuelan people," US Treasury Secretary Steven Mnuchin said in a statement announcing the action.
He added: "By sanctioning Maduro, the United States makes clear our opposition to the policies of his regime and our support for the people of Venezuela who seek to return their country to a full and prosperous democracy."
National security adviser H.R. McMaster said: "Maduro is not just a bad leader. He is now a dictator."
He added: "Recent actions culminating in yesterday's seizure of absolute power through the sham election of the National Constituent Assembly represent a very serious blow to democracy in our hemisphere."
Sunday's vote in Venezuela to elect a Constituent Assembly that would supersede the country's opposition-controlled National Assembly earned international scorn, with the US leading a group of nations refusing to recognize the election.
Opposition parties sat out the election, saying it was rigged as part of a naked power grab by Maduro, leaving all 545 of the assembly's seats for the socialists and their allies.
The order does not impose restrictions on Venezuela's oil exports to the US.
A statement from the US Treasury said: "As a result of today's actions, all assets of Nicolas Maduro subject to US jurisdiction are frozen, and US persons are prohibited from dealing with him."
"Under Maduro, the Venezuelan government has deliberately and repeatedly abused the rights of citizens through the use of violence, repression and criminalisation of demonstrations," it alleged.
"At his direction, the regime's security forces have systematically repressed and criminalized opposition parties through arbitrary detention, military prosecution of civilians and the excessive use of force against demonstrators.
"Any member of the opposition or critic of the regime risks being detained, imprisoned, assaulted, tortured and assassinated."
The statement urged those Venezuelans elected to the controversial Constituent Assembly not to take their seats, and warned that any who support the imposition of "an authoritarian regime" will also face sanctions.
Meanwhile, anti-government demonstrators barricaded streets and promised a fresh wave of protests against President Maduro.
At least 10 people were killed in weekend protests against the unpopular leader, who insists the country's newly elected constituent assembly will bring peace after four months of protests that have killed more than 120 people.
