The Venezuelan government has detained opposition leader Juan Guaido's chief of staff in a pre-dawn raid, accusing him of "terrorism".
US Secretary of State Mike Pompeo called on Twitter for Roberto Marrero's immediate release.
The detention is also testing repeated US warnings that President Nicolas Maduro should not go after his rivals.
"We will hold accountable those involved," Mr Pompeo said.
Mr Guaido invoked the constitution in January to assume the interim presidency after declaring Mr Maduro's 2018 re-election a fraud.
He has been recognised by the US and dozens of other western nations as the country's legitimate leader.
Mr Maduro, who has overseen a dramatic collapse of the Opec nation's economy, has called Mr Guaido a puppet of the US and said he should "face justice", but has not ordered his arrest.
Top US officials have repeatedly warned Mr Maduro not to touch Mr Guaido or his inner circle, and threatened ever harsher sanctions to further isolate the president and cut off his sources of revenue.
US National Security Adviser John Bolton told Mr Maduro this month that Mr Guaido's safety must be guaranteed and that the world was watching.
Mr Guaido said the raids by the Sebin intelligence service on the homes of Mr Marrero and another opposition legislator, Sergio Vergara, showed Mr Maduro's weakness.
He said attempts to intimidate him would not derail the opposition campaign.
"As they cannot take the interim president prisoner, so they seek out people closest to him, threaten relatives, carry out kidnappings," Mr Guaido said.
Mr Marrero recorded a voice message as the agents were trying to enter his home in the upscale Caracas neighbourhood of Las Mercedes.
"I am in my house and the Sebin is here," he said. "Unfortunately, they have come for me. Keep up the fight, don't stop and look after Mr Guaido."
Mr Vergara, Mr Marrero's neighbour, said about 40 armed agents forced their way into their homes and spent three hours inside.
The Sebin agents left with Mr Marrero and Mr Vergara's driver, the legislator said on his Twitter account.
Mr Guaido said that Mr Marrero had told Mr Vergara that agents had planted two rifles and a grenade in his house.
Venezuela's Information Ministry did not immediately respond to a request for comment.
