Maduro himself claims that a "high-ranking official" within the US federal police (FBI) and a high-ranking American military officer are among the seven foreign "mercenaries" arrested in recent days.
Maduro has repeatedly claimed that the US is trying to overthrow him. The latest development comes ahead of his inauguration for a third term as president on Friday.
The opposition condemns the increasing number of arrests.
According to a coalition of opposition parties, Enrique Márquez, who ran against Maduro in the summer election, has been "arbitrarily detained".
The organization Espacio Público, which works with press freedom, reports that it has lost contact with its leader Carlos Correa. Eyewitness accounts claim that he was stopped in central Caracas by men believed to be from the regime.
The authorities have not confirmed the arrests of either Márquez or Correa.
Edmundo González Urrutia, whom the opposition considers the winner of the presidential election and who is now in exile, claims that his son-in-law has been "kidnapped" by men in black.
Opposition politician María Corina Machado says that her family has also been targeted, and that her 84-year-old mother's house has been surrounded.
The EU, USA, UN, and several neighboring countries have expressed doubts about Maduro's victory in the presidential election in July and point out that the election data has not been fully disclosed.
