A top Venezuelan government official has confirmed that the uncle of opposition leader Juan Guaidó is being held on suspicion of smuggling "dangerous material" into the country.
Juan José Márquez was travelling with Mr Guaidó from Lisbon to Caracas by plane and vanished on Tuesday after being stopped by tax agency personnel.
Mr Guaidó called it a "cowardly move".
The official did not say how Mr Márquez would have managed to smuggle the items onto an international flight.
Mr Márquez is not the first person with links to Mr Guaidó to be arrested. His chief of staff, Roberto Marrero, was detained last year and remains in prison.
What does the government say?
Diosdado Cabello, widely considered the second most powerful man in President Nicolás Maduro's government, spoke of Mr Márquez's arrest on his weekly TV programme.
"No, he's not forcibly disappeared, he is being held for bringing in forbidden substances on a flight," Mr Cabello alleged.
Mr Cabello showed photos of a bulletproof vest and what he said was explosive material which Mr Márquez had allegedly tried to smuggle into Venezuela.
"He carried tactical flashlights which contained, hidden in the battery compartment, chemical substances of an explosive nature, presumably C-4 synthetic explosives," he said.
Mr Cabello did not say how Mr Márquez would have managed to smuggle such items onto an international commercial flight from Lisbon in Portugal to Caracas.
"No doubt they [the opposition] are going to shout about this [arrest]," Mr Cabello said. "Let them, because if this arrest is going to save the life of one Venezuelan, so be it. Enough is enough, who knows who he was going to use those explosives on?" he asked.
"Tomorrow, they're probably going to say we made this up and that he is a little saint."
Mr Cabello said Mr Márquez would not be released and a court later ruled that he should continue to be held.
Local media reported that Mr Márquez was driven away from the court in a car belonging to Venezuela's military counterintelligence agency (DGCIM).
What's the reaction been?
Mr Guaidó called President Maduro a "coward, who does not show his face, who does not dare to step into a public square without security... but mounts an attack on my family".
"It's a crime, it's a kidnapping, he's been forcibly disappeared by the dictatorship, this cowardly dictatorship."
Mr Márquez's wife said her husband, a pilot, had nothing to do with politics and had only accompanied his nephew on the flight out of concern for his safety.
She said her husband had been carrying a "protective vest", which she said had been justified in light of the attacks that supporters of Mr Guaidó and reporters had suffered as they welcomed him at the airport.
The arrest has also been denounced by the US state department with the acting assistant secretary for Western Hemisphere Affairs denouncing it as "a kidnapping".
Kidnapping interim president @JGuaido’s relatives only demonstrates that the dictatorship is weak and desperate.— Michael G. Kozak (@WHAAsstSecty) February 12, 2020
We demand the immediate release of Juan José Márquez unharmed. #Democracy cannot be intimidated, this must stop! https://t.co/R6VcDvJu8x
What's the background?
Juan Guaidó, 36, has been a thorn in the side of President Maduro for the past 13 months, ever since the former declared himself interim president and promised to oust Mr Maduro from office.
Mr Guaidó argues that the 2018 re-election of Mr Maduro was illegitimate and that he as leader of the National Assembly therefore had the duty to take over as interim president.
He promised an "end to the usurpation [of President Maduro], [to create] a transitional government and have free elections".
While Mr Guaidó was recognised as Venezuela's legitimate leader by more than 50 countries he was not able to wrest control of the executive from Mr Maduro, who continues to have the backing of the country's influential military.
In January, Mr Guaidó defied a travel ban against him to go on a three-week tour to gain support for further sanctions on the Maduro government.
He held talks with European leaders including Germany's Angela Merkel, France's Emmanuel Macron and British Prime Minister Boris Johnson. In the US, he attended the State of the Union speech as a special guest of President Donald Trump.
There was much expectation about how Mr Guaidó would return to Venezuela and if he would be arrested.
On Tuesday, he returned on a TAP airlines flight from Lisbon and while the immigration official seized his identity card, he was allowed to enter the country unhindered.