Leopoldo Lopez and Antonio Ledezma were both already under house arrest when they were taken into custody, and the Venezuelan Supreme Court later alleged that they had been planning to flee, either into hiding in Venezuela or abroad.
"The United States condemns the actions of the Maduro dictatorship," Trump said in a statement issued by the White House.
"Mr Lopez and Mr Ledezma are political prisoners being held illegally by the regime."
The senior US diplomat for the Americas, Acting Assistant Secretary Antonio Francisco Palmieri, tweeted that this was "another step in the wrong direction for Venezuela."
"This action is further evidence of the Maduro regime's authoritarianism," he added, speaking one day after senior US officials declared for the first time that they now regard Maduro, who was elected as a socialist, as a dictator.
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State Department spokeswoman Heather Nauert also condemned the detentions, saying in a tweet that the United States is "deeply concerned."
On Monday, the US Treasury slapped sanctions on Maduro, and National Security Adviser HR McMaster compared him to notorious strongmen Bashar al-Assad of Syria, Robert Mugabe of Zimbabwe and North Korea's Kim Jong-Un.
"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," Trump said.
"We reiterate our call for the immediate and unconditional release of all political prisoners."
The raids, carried out in the dead of night, came just one day before a new assembly elected on Sunday is supposed to take office, superseding the opposition-controlled legislature.
In a statement, the Venezuelan Supreme Court said Lopez and Ledezma were sent back to prison because they had violated the terms of their house arrest by making political statements.
Authorities acted with urgency, it said, because they had received intelligence that the pair "had a plan to flee" -- something the men's lawyers vehemently denied.
In a video he pre-recorded in case he was sent back to jail, Lopez urged his supporters to keep fighting Maduro's government.
"If you're seeing this video, it's because they illegally and unjustly came and returned me to prison. I'm a political prisoner," he said.
"We must not give up the fight. We must never surrender. We must not tire of demanding a better Venezuela."
Lopez, the Harvard-educated founder of the Popular Will party, also announced that his wife, Lilian Tintori, was pregnant, calling it "the best news" since he was arrested in 2014, and "one more reason to fight for Venezuela."
The men are two of Venezuela's most high-profile opposition leaders. Both had called for a boycott of Sunday's vote for an all-powerful "constituent assembly" tasked with rewriting the constitution.
The United States, which has already slapped sanctions on Maduro and top officials, was scathing in its reaction to the latest news.
"Overnight, the regime of the Venezuelan dictator Maduro detained two leading opposition figures following its outrageous seizure of power through sham elections this weekend," said White House Press Secretary Sarah Sanders.
"The United States condemns the actions of the Maduro dictatorship and we hold the regime responsible."
UN Secretary-General Antonio Guterres urged Maduro's administration to "lower tensions" and "find avenues for political dialogue," an appeal echoed by EU diplomatic chief Federica Mogherini's spokeswoman.
Lopez and Ledezma were picked up by the intelligence service known by its acronym Sebin, their families said, adding that they held Maduro responsible for the men's lives.
Families in the dark
"They just took Leopoldo away. We do not know where he is or where they are taking him," Tintori said on Twitter.
She released home security camera footage in which four uniformed police officers and three others in civilian garb are seen putting her husband into a car and taking off, with other cars escorting them.
Ledezma's family also released a cell phone video in which the mayor is seen being hauled from home in a pair of blue pyjamas as his neighbors scream.
Lopez, 46, was transferred to house arrest in July after serving three years and five months in prison as part of a 14-year term. He had been convicted of instigating violence during protests against Maduro in 2014 that left 43 people dead.
Ledezma, 62, was arrested in February 2015 on charges of conspiracy and racketeering and was placed under house arrest three months later for health reasons.
Opposition lawmaker Freddy Guevara said the re-arrests were aimed at "frightening us and demoralizing us."
Four months of street demonstrations since April against Maduro have left more than 120 people dead, including 10 over the weekend.
The new constituent assembly is to start working on Wednesday. It is made up only of members of Maduro's Socialist party, including his own wife. The opposition has called for protests against the inauguration.
'Imperial orders'
Maduro has dismissed the US sanctions and criticism, retorting that he will not heed "imperial orders."
Latin American nations including Colombia, Mexico and Peru joined the US in saying they did not recognize the results of Sunday's election, while Brazil, Costa Rica and Panama joined the condemnation of Lopez and Ledezma's arrest.
Officials say more than 40 percent of Venezuela's 20 million voters cast ballots Sunday.
The opposition says turnout was closer to 12 percent -- on a par with the population of state employees, who were under major pressure to vote.
According to polling firm Datanalisis, more than 70 percent of Venezuelans oppose the new assembly.
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