Agents from Venezuela's Sebin intelligence service on Tuesday raided the offices of Juan Guaido while the opposition leader was travelling in Europe, an opposition lawmaker said.
"We have just confirmed that Sebin officers are inside the office of president Guaido," lawmaker Delsa Solorzano told reporters after speaking to security guards at Caracas's Zurich Tower.
National Assembly leader Guaido has been recognised as president by the United States and more than 50 other countries in his year-long power struggle with socialist President Nicolas Maduro, who earlier this year tried to block Guaido's re-election as Assembly president.
The opposition said one of their lawmakers, Ismael Leon, was also arrested on Tuesday on his way to the National Assembly.
"Cowardly dictatorship! While I am away, consolidating support to overcome the tragedy that we Venezuelans are enduring, they shamelessly show what they are all about," Guaido, visiting London, wrote on Twitter.
The tower where the offices are located had been surrounded in the afternoon by hooded and armed Sebin officers dressed in black.
The offices were empty when the search began, the opposition said, adding that several lawmakers had been unable to gain access.
"Whatever procedure is under way, they are carrying it out without authorisation... without witnesses, and they have no search warrant," Solorzano said outside the building.
Lawmaker Angel Torres said the agents "abruptly entered" the offices, amid opposition fears they could plant false evidence.
Venezuelan authorities made no comment on the report of the arrest of Leon.
The 36-year-old Guaido has defied a travel ban to fly to London for talks with Prime Minister Boris Johnson as part of a European trip which will include a visit to the World Economic Forum in Davos, Switzerland.
On Monday he visited Bogota, Colombia and met US Secretary of State Mike Pompeo, who promised Guaido more US support in his effort to unseat Maduro.
Despite international backing, efforts to oust Maduro have stalled and he retains the support of the powerful armed forces, as well as that of allies China, Russia and Cuba.
