Venezuelan intelligence officers Thursday arrested the chief of staff of Juan Guaido, the opposition leader recognized by the US and other countries as interim leader, Guaido and the opposition-ruled congress said on Twitter.
Roberto Marrero was grabbed by SEBIN officers when they staged a pre-dawn raid on his Caracas home, according to Guaido and a recorded voice message by Marrero published on social media.
The United States has repeatedly warned Venezuelan President Nicolas Maduro's government against arresting Guaido or his aides, and Washington's Secretary of State Mike Pompeo quickly called for Marrero's release.
"They have grabbed Roberto Marrero, my chief of staff. He yelled out that they planted two rifles and a grenade," Guaido tweeted. "The raid happened at around 2:00 am (0600 GMT). We don't know his whereabouts. He must be released immediately."
Maduro and Guaido both claim to be legitimate leader of Venezuela.
Maduro, 56, retains the loyalty of military brass and has control of state apparatus.
Guaido, 35, declared himself interim president on January 23 and has the backing of the US and more than 50 other countries, mostly in Latin America and the US.
"The United States condemns raids by Maduro's security services and detention of Roberto Marrero, Chief of Staff to Interim President @jguaido. We call for his immediate release. We will hold accountable those involved," Pompeo said on Twitter.
US President Donald Trump has said, as recently as Tuesday, that "all options" remain on the table in his drive to bring down Maduro, implying military action if he deemed it necessary.
So far, however, the power struggle in Venezuela has become bogged down in an impasse, with Maduro railing daily about the US "imperialists" trying to dislodge him and Guaido touring the country to rally supporters and pledge he'll be taking over "very soon."
- 'Ransacked' home -
Maduro's forces have reinforced obstacles blocking a border bridge linking Venezuela and Colombia to prevent Guaido's supporters trucking in US aid stockpiled on the other side. Extra shipping containers and concrete blocks have been moved into place on the bridge.
As well as seizing Marrero early Thursday, SEBIN officers raided the next-door home of an opposition lawmaker, Sergio Vergara, the opposition-ruled congress -- the National Assembly -- tweeted.
Vergara was not arrested. He told reporters that he saw Marrero bundled off into the street.
He said around 15 SEBIN officers "ransacked" his own home for around two hours, while asking where to find Marrero, a lawyer who works in the National Assembly.
"They started to bash on the door of Roberto Marrero's place, which is a few meters (yards) from my door, until they were able to get inside," he said.
"The dictatorship is abducting citizens," he added.
- US warning -
The United States has cautioned Maduro to not lay a finger on Guaido or National Assembly deputies or risk unspecified repercussions.
Trump's National Security Advisor John Bolton in January tweeted: "Any violence and intimidation against US diplomatic personnel, Venezuela's democratic leader, Juan Guaido, or the National Assembly itself would represent a grave assault on the rule of law and will be met with a significant response."
The United States has since withdrawn all its diplomats from Venezuela.
In just over a month, on April 28, increasingly harsh US sanctions on Venezuela will jump up a critical level with a ban on all oil sales to the United States, Venezuela's main crude buyer.
The step is expected to worsen already dire economic conditions ravaging Venezuela, a once-wealthy South American nation that has become impoverished under Maduro.