Two of Venezuela's leading opposition figures have been taken from their homes in the middle of the night by state security agents.
The developments appeared to be President Nicolas Maduro's first moves against his enemies since a widely denounced vote giving his government nearly unlimited powers.
Relatives and allies of Leopoldo Lopez and former Caracas mayor Antonio Ledezma reported on social media that both had been detained.
Mr Lopez's wife posted what appeared to be video of him being taken from their home after midnight.
"They've just taken Leopoldo from the house," Lilian Tintori wrote on Twitter. "We don't know where he is or where they're taking him."
Allies of Mr Ledezma posted video of a man who appeared to be the opposition leader being taken by state security as a woman screams for help from neighbours.
"They're taking Ledezma!" she cries. "It's a dictatorship!"
A lawyer for Mr Lopez told the Exitos radio station that he was returned to the same military prison from which he had been recently released to house arrest.
Juan Carlos Gutierrez said the government's decision to return Mr Lopez to prison is "completely arbitrary" and said Mr Lopez had obeyed the conditions imposed on his house arrest.
Mr Lopez had been released from Ramo Verde on July 8 after serving three years of a 13-year sentence for inciting violence at opposition rallies.
Many human rights groups considered him a political prisoner.
It is not clear if Mr Ledezma was also taken to prison. He was also detained in 2015 and has been under house arrest.
Both leaders had recently posted videos online denouncing Mr Maduro's decision to hold a vote for a constitutional assembly with the power to overhaul Venezuela's political system.
Mr Maduro said on Monday evening he had no intention of deviating from his plans to rewrite the constitution and go after a string of enemies, from independent Venezuelan news channels to gunmen he claimed were sent by neighbouring Colombia to disrupt the vote as part of an international conspiracy led by the man he calls "Emperor Donald Trump".
"They don't intimidate me. The threats and sanctions of the empire don't intimidate me for a moment," Mr Maduro said on national television.
"I don't listen to orders from the empire, not now or ever ... Bring on more sanctions, Donald Trump."
A few hours earlier, Washington added Mr Maduro to a steadily growing list of high-ranking Venezuelan officials targeted by financial sanctions, escalating a tactic that has so far failed to alter his socialist government's behaviour.
For now, the Trump administration has not delivered on threats to sanction Venezuela's oil industry, which could undermine Mr Maduro's government but raise US fuel prices and deepen the humanitarian crisis in Venezuela.
The sanctions came after electoral authorities said more than eight million people voted on Sunday to create the constitutional assembly - a turnout doubted by independent analysts, while the election was labelled illegitimate by leaders across the Americas and Europe.
Venezuela's National Electoral Council said turnout in Sunday's vote was 41.53%, or 8,089,320 people.
The result would mean the ruling party won more support than it had in any national election since 2013, despite a struggling economy, spiralling inflation, shortages of medicine and malnutrition.
Opinion polls had said some 85% of Venezuelans disapproved of the constitutional assembly and similar numbers disapproved of Mr Maduro's overall performance.
Opposition leaders estimated the real turnout at less than half the government's claim in a vote watched by government-allied observers but no internationally recognised poll monitors.
An exit poll based on surveys from 110 voting centres by New York investment bank Torino Capital and a Venezuela public opinion company estimated 3.6 million people voted, or about 18.5% of registered voters.
The national electoral council's vote counts in the past had been seen as reliable and generally accurate, but the widely mocked announcement appeared certain to escalate the polarisation and political conflict paralysing the country.
The constituent assembly will have the task of rewriting the country's constitution and will have powers above and beyond other state institutions, including the opposition-controlled congress.
Mr Maduro has said the new assembly will begin to govern within a week.
Among other measures, he said he would use the assembly's powers to bar opposition candidates from running in gubernatorial elections in December unless they sit with his party to negotiate an end to hostilities that have generated four months of protests that have killed at least 120 and wounded nearly 2,000.
Along with the US, the European Union and nations including Argentina, Canada, Colombia, Mexico, Panama, Paraguay, Spain and Britain criticised Sunday's vote.
Mr Maduro said he had received congratulations from the governments of Cuba, Bolivia and Nicaragua, among others.
He called the constitutional assembly in May after a month of protests against his government, which has overseen Venezuela's descent into a devastating crisis during its four years in power.
Due to plunging oil prices and widespread corruption and mismanagement, Venezuela's inflation and homicide rates are among the world's highest, and widespread shortages of food and medicine have citizens dying of preventable illnesses and rooting through rubbish to feed themselves.