Leopoldo López and Antonio Ledezma were under house arrest after allegedly inciting violence during protests against the government in 2014 in which three people died. Others said they were warned subsidized food bags would be taken away.
Maduro said 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 global conspiracy led by the man he calls "Emperor Donald Trump".
"They're taking Ledezma!" she cries.
Local media posted mobile phone footage of Mr Ledezma, wearing pyjamas, being taken forcibly from his home as a woman shouts "dictatorship!"
The opposition urged residents to boycott the vote.
Cuba said only the Venezuelan people could decide how to overcome their problems and reiterated its solidarity with them and their government.
As part of what are expected to be a series of escalating sanctions, the Treasury Department added Maduro to its growing list of sanctioned current and former members of the Venezuelan government and military.
Canada also denounced the Venezuelan government's action in creating the constituent assembly, referring to it as another step down the path to institutionalize authoritarian rule in the country.
Kathy Griffin just shaved her head for the most lovely of reasons
Griffin's sister , Joyce , has been diagnosed with cancer and is now undergoing treatment for the invasive disease. But regardless of what anyone may think her her, Kathy Griffin certainly never does anything in half measures.
Venezuela's National Electoral Council said that just a little over eight million ballots had been cast for members of the new assembly created to change the country's constitution, a turnout of 41.53% of the electorate, amid violence that reportedly resulted in at least ten deaths across the country. The threats and sanctions from the imperialists don't intimidate me one bit in this world. "By sanctioning Maduro, the United States makes clear our opposition to the policies of his regime and the support for the people of Venezuela, who strive to return their country to a full and prosperous democracy", said the Treasury Secretary.
"I don't obey orders from foreign governments and never will..."
"The latest sanctions are more symbolic than anything else, as the oil sector remains exempted", said UBS oil analyst Giovanni Staunovo.
Maduro says a new constitution is the only way to end such conflicts.
On Sunday, the vote rendered a victory for Venezuelan president Nicolas Maduro.
The National Electoral Council claimed more than 40% of Venezuela's 20 million voters had cast ballots on Sunday.
A spokeswoman for the European Commission said: "A Constituent Assembly, elected under doubtful and often violent circumstances, can not be part of the solution". However, imposing sanctions on a head of state is rare and can be symbolically powerful, leading other countries to similarly shun such a leader.
Along with the USA, the European Union and nations including Argentina, Canada, Colombia, Mexico, Panama, Paraguay, Spain and Britain criticized Sunday's vote.