CARACAS, Venezuela — The Venezuelan authorities on Tuesday began investigating the leader of the opposition and detained a prominent journalist, claiming they had played a role in the supposed sabotage of the country’s electrical system. Government critics said it was an effort to deflect blame for a four-day national power failure.

Attorney General Tarek Saab accused Juan Guaidó, the opposition leader who leads a parallel government backed by the United States and about 50 other countries, of masterminding the blackout. He provided no evidence.

Hours earlier, intelligence police detained a prominent radio producer and social media activist, Luis Carlos Díaz, as he rode his bike home from work, according to his wife, Naky Soto. He was charged with inciting violence and freed on parole a day later.

Mr. Guaidó, who has not been detained or charged, brushed off the accusations as baseless, and spent Tuesday addressing small flash protests around the capital, Caracas.