In a statement, President Trump also called for the immediate and unconditional release of all political prisoners.
Leopoldo López and Antonio Ledezma, who had been under house arrest, were taken to a military prison on August 1.
This came after July 30 controversial vote for a constitutional assembly.
At least 10 people were killed, as the opposition boycotted the election.
President Nicolas Maduro said the poll was a “vote for the revolution”, arguing that the move would create peace and foster dialogue by bringing together different sectors of Venezuela’s polarized society.
The opposition said on August 1 that it had decided to hold a demonstration against the new assembly on August 3.
Opposition lawmaker Freddy Guevara tweeted the rally would be held on “the day the dictatorship plans to install the fraudulent assembly”.
It had originally been thought that the government would open the constituent assembly on Wednesday, thereby evicting the opposition-controlled National Assembly.
In his statement, President Trump said the US condemned the actions of the “Maduro dictatorship”.
Leopoldo López and Antonio Ledezma were “political prisoners being held illegally by the regime”, he added.
“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. López, Mr. Ledezma and any others seized.”
Earlier, Secretary of State Rex Tillerson described the re-arrest of Leopoldo López and Antonio Ledezma as “very alarming”.
“The situation from a humanitarian standpoint is already becoming dire,” he said.
“We are evaluating all of our policy options as to what can we do to create a change of conditions, where either Maduro decides he doesn’t have a future and wants to leave of his own accord, or we can return the government processes back to their constitution.”
US Treasury Secretary Steven Mnuchin announced on July 31 that the US had imposed sanctions on President Maduro.
Leopoldo López was taken from his home at 12:27 local time on August 1, his wife, Lilian Tintori, wrote on Twitter.
A video posted showed Leopoldo López being taken away by members of the Venezuelan intelligence service, Sebin.
The daughter of Antonio Ledezma, Vanessa Ledezma, also posted a video of her father, wearing pajamas, being taken away by the Sebin.
Both Leopoldo López and Antonio Ledezma were key figures in the wave of protests which swept through Venezuela in 2014 in which 43 people from both sides of the political divide were killed.
They have played a less prominent role in the most recent protests because they have been under house arrest but their video messages still get reported and shared widely on opposition websites.
The US House Ethics Committee has voted to release its report on former Republican Representative…
ABC News has agreed to pay $15 million to President-elect Donald Trump to settle a…
South Korea’s parliament has voted to impeach President Yoon Suk Yeol over his failed attempt…
Israeli war planes have carried out more than 100 air strikes in Syria on December…
President-elect Donald Trump has threatened to impose 100% tariffs on the BRICS countries if they…
Syrian troops have withdrawn from the city of Aleppo following an offensive by rebels opposed…
