"I am deeply concerned that opposition leaders Leopoldo Lopez and Antonio Ledezma have again been taken into custody by Venezuelan authorities after their house arrest was revoked", said UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein in a statement issued by his Office (OHCHR). "We don't know where he is or where they're taking him".
"We reiterate our call for the immediate and unconditional release of all political prisoners".
Crude futures were ticking higher early Tuesday in Asia, carried by a bullish momentum that led to the sixth straight session of gains Monday, despite the threat of a U.S. oil embargo or financial sanctions against Venezuela retreating into the background for the time being.
The United States has called the vote a sham and Freeland says the existing national assembly is the only legitimately elected representative body in Venezuela.
Violence in the South American state has seen the families of British embassy staff withdrawn with Boris Johnson, the Foreign Secretary, accusing Mr Maduro of behaving like the "dictator of an evil regime" as the President appeared to crack down on his enemies after a widely disputed vote.
Leopoldo and Ledezma had denounced Sunday's vote to change the constitution and grant Maduro new powers, labeling the poll undemocratic. The new political assembly -installed by the political mechanisms of authoritarianism - will supersede all other political institutions in Venezuela.
The sanction package that is being considered in Washington may include a ban on access to the USA banking system for the Venezuelan state-owned oil and natural gas company PDVSA, sources familiar with the White House discussion told CNBC.
Last week, USA refiners Valero Energy Corp and Marathon Petroleum Corp said they planned to run more light and sweet crudes this quarter, continuing a trend away from heavy, sour crudes supplied by Venezuela and other OPEC producers.
Mr Lopez's lawyer said he and Mr Ledezma were both in Ramo Verde prison, a decrepit penitentiary next to a slum in Los Teques, about an hour's drive from the capital.
Ledezma was also detained in 2015 and has been under house arrest. Therefore, it goes without saying that more people will die - and this applies to people demonstrating and based on an infrastructure that can't provide an adequate health care system.
About 120 people have been killed in more than four months of anti-government street protests, including at least 10 during Sunday's vote.