Porto vowed to "cooperate with justice" after confirming they were subject to searches by the Central Criminal Investigation Court as part of a wide-scale probe into alleged tax fraud and money laundering.
A statement from Portugal's Attorney General's Office on Wednesday said 76 searches were taking place, "including house searches, namely in several football clubs, respective companies and managers, law firms and intermediary agents".
The release added the operation relates to an investigation that began in 2015, probing alleged actions "aimed at avoiding the payment of tax benefits owed to the Portuguese states through concealment or alteration values".
Porto confirmed the club and the chairman of the board of directors, Jorge Nuno Pinto da Costa, were subject to searches and promised to aid the investigation.
"FC Porto - Football, SAD, FC Porto Comercial and its chairman of the Board of Directors confirm that they were targets of searches promoted by the Central Criminal Investigation Court," a club statement read.
"As always, they're cooperating with justice."
A number of other Portuguese top-flight clubs are said to be involved in the investigation.
