WASHINGTON (REUTERS) - The United States on Thursday (March 22) warned of "consequences" against Venezuelan President Nicolas Maduro's security services if they continue with a crackdown against opposition leader Juan Guaido and his aides.

A US State Department spokesman called for the release of Guaido's chief of staff, Robert Marrero, who was arrested in a pre-dawn raid.

"The detentions show Maduro is betting on repression to hold on to power. It will not stand," the spokesman said. "There will be consequences for a continued crackdown."

The spokesman said the arrests were facilitated by Venezuelan Judge Carol Padilla, prosecutors Farid Mora Salcedo and Dinora Bustamante, and Sebin intelligence officials Danny Contreras and Angel Flores, and added the United States and regional partners were considering next steps.

The arrest was denounced earlier on Thursday by Guaido as a "vile, vulgar kidnapping." He said "we won't be intimidated".

One opposition lawmaker who is a neighbour of Marrero's and whose home was also searched, Sergio Vergara, told reporters that Marrero had yelled out during his arrest that Sebin intelligence officers had planted two assault rifles and a grenade in his place as a pretext.

"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," US Secretary of State Mike Pompeo said on Twitter.

"Maduro has made another big mistake. The illegitimate arrest of Roberto Marrero, Interim President Juan Guaido's aide, will not go unanswered. He should be released immediately and his safety guaranteed," added National Security Advisor John Bolton.

Venezuelan Interior Minister Nestor Reverol accused Marrero, a 49-year-old lawyer, of leading a "terrorist cell" bent on attacking the government's leadership with the help of Colombian and Central American mercenaries as well as "acts of sabotage on public services to create chaos."

He said weapons and foreign cash were found in a predawn raid on Marrero's home.

He added that Marrero's 34-year-old bodyguard, Luis Paez, was also arrested and faced the same charges and a search was on for "identified" collaborators.

Later, President Nicolas Maduro said he would "not be afraid to fight terrorist groups to put them in jail".

The development triggered alarm internationally. The European Union, and a grouping of Latin American nations plus Canada all denounced Marrero's arrest and demanded his immediate release.

A UN spokesman on Thursday expressed "concern" at the arrest, urging "all actors in Venezuela to take immediate steps to lower tensions and refrain from any action that could lead to further escalation." .

Maduro and Guaido both claim to be Venezuela's legitimate leader but Maduro, 56, retains the loyalty of the 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.

So far their power struggle has hit an impasse, with Maduro railing daily about the US "imperialists" trying to dislodge him and Guaido touring the country to rally supporters and pledging he'll be taking over "very soon".

Maduro's forces have reinforced obstacles blocking a border bridge linking Venezuela and Colombia to prevent Guaido's supporters trucking in US aid. Extra shipping containers and concrete blocks have been moved into place on the bridge.

Vergara said Thursday's early morning raid was carried out by around 15 officers of the Bolivarian Intelligence Service, or Sebin.

He said he was thrown to the floor and his own place "ransacked" for around two hours before the officers went a short distance to bang on Marrero's door and enter his apartment.

"The dictatorship is abducting citizens," Vergara said.

The Lima Group, which includes Argentina, Brazil, Canada, Chile, Colombia, Costa Rica, Guatemala, Panama and Peru, issued a statement "calling for Nicolas Maduro's illegitimate and dictatorial regime to immediately release Mr Marrero" and leave Vergara alone.

"We demand the end of harassment of Venezuelans upholding democracy and the systematic practice of arbitrary detention and torture in Venezuela," said the statement issued by Peru's foreign ministry.