The investigation relates to “suspicions of aggravated fraud”, the prosecutor said.
"Portugal Telecom will co-operate fully with the authorities," a spokeswoman for Portugal Telecom told AFP.
Investigators have also searched the offices of auditing group PwC, officials said.
The search comes just days before Portugal Telecom is expected to respond formally next week to the sale of its Portuguese assets to Altice, the parent company of France’s Numericable-SFR.
Last year Portugal Telecom loaned €897 million ($1.2 billion) to Rioforte, a holding company of the Espirito Santo banking family, just months before Rioforte and the family went bankrupt. Portugal Telecom’s chairman Henrique Granadeiro resigned shortly after. Portugal Telecom said at the time that its executive board had never approved or discussed any investment in debt issued by Rioforte.
