The US has condemned what it calls "the Maduro dictatorship" over the arrests of two top opposition leaders in Venezuela following a highly-controversial election .
Leopoldo Lopez and former Caracas mayor Antonio Ledezma had been under house arrest but security force officers then allegedly took them to a military prison.
In a statement, the White House called them political prisoners and says they are "being held illegally by the regime" led by Venezuelan President Nicolas Maduro.
The statement adds: "The United States holds Maduro who publicly announced just hours earlier that he would move against his political opposition, personally responsible for the health and safety of Mr. Lopez, Mr. Ledezma, and any others seized."
It calls for Venezuela's government to immediately free all political prisoners.
Venezuela's vice president announced on the country's state television that the newly elected constituent assembly that could dramatically reshape the nation's government will be convening "within hours."
On Tuesday Boris Johnson said Venezuela's president is behaving like the "dictator of an evil regime".
- Video report by ITV News Correspondent Neil Connery
Officials say that eight million people voted to grant president Nicolas Maduro nearly unlimited powers on Sunday - but opponents reject this figure and fear democracy is being eroded.
Fifteen people were reported to have been killed in violence on the day of voting, with Britain and the US both condemning the election after as a "sham".
The Foreign Secretary also accused Mr Maduro of taking political prisoners following the arrests of the opposition.
Relatives of Mr Lopez and Mr Ledezma reported on Tuesday that the pair had been detained by the state.
Both had recently posted videos online criticising Mr Maduro's decision to hold the election, the run up to which has seen more than 100 people killed.
Mr Johnson tweeted: "Maduro acting like a dictator of an evil regime and has destroyed Venezuelan economy, eroded human rights + imprisoned thousands".
In another post, he added: "Hundreds have died during protests against Maduro's actions. Political prisoners must be released + rights, freedoms + democracy respected."
The Venezuelan election was called in May following a month of protests against Mr Maduro's government.
Plunging oil prices and widespread corruption have left the formerly prosperous nation struggling with widespread shortages of food and medicine.
Opposition parties boycotted the election, which saw 5,500 ruling-party candidates run for 545 seats.
The US reacted to the election by imposing financial sanctions on Mr Maduro.
He responded on national television, saying that the sanctions "don't intimidate me for a moment", adding: "I don't listen to orders from the empire, not now or ever".