Relatives and allies of Lopez and former Caracas Mayor Ledezma earlier reported on social media that both had been detained. 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 Ledezma posted video of a man who appeared to be the opposition leader being taken by state security as a woman screams for help for neighbors.
"They're taking Ledezma!" she cries. "It's a dictatorship!"
Attorney Juan Carlos Gutierrez said the government's decision to return Lopez to prison was "completely arbitrary" and said Lopez had obeyed the conditions imposed on his house arrest and had never had plans to flee.
Lopez had been released from the Ramo Verde military prison 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.
Ledezma was also detained in 2015 and has been under house arrest. Both leaders recently posted videos online denouncing Maduro's decision to hold a vote for a constitutional assembly with the power to overhaul Venezuela's political system.
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 neighboring 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," 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 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 behavior. For now, the Trump administration has not delivered on threats to sanction Venezuela's oil industry, which could undermine Maduro's government but raise U.S. gas prices and deepen the humanitarian crisis here.
The sanctions came after electoral authorities said more than 8 million people voted Sunday to create the constitutional assembly a turnout doubted by independent analysts while the election was labeled illegitimate by leaders across the Americans and Europe.
Venezuela's National Electoral Council said turnout in Sunday's vote was 41.53 percent, 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 cratering economy, spiraling inflation, shortages of medicine and malnutrition. Opinion polls had said some 85 percent of Venezuelans disapproved of the constitutional assembly and similar numbers disapproved of 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 recognized poll monitors.
An exit poll based on surveys from 110 voting centers by New York investment bank Torino Capital and a Venezuela public opinion company estimated 3.6 million people voted, or about 18.5 percent of registered voters.
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.
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 U.S., the European Union and nations including Argentina, Canada, Colombia, Mexico, Panama, Paraguay, Spain and Britain criticized Sunday's vote. Maduro said he had received congratulations from the governments of Cuba, Bolivia and Nicaragua, among others.
Maduro 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 trash to feed themselves.