Investigators swooped on a string of top football clubs in Portugal on Wednesday as part of a tax evasion probe, the prosecutor's office announced.
Benfica and Porto confirmed searches and declared themselves ready to collaborate with the authorities.
"Porto... Confirm that they were the target of searches carried out by the Central Court of Criminal Investigation," said Porto in a statement.
"As always, they are collaborating with the legal authorities."
Sporting Lisbon and Braga, as well as the offices of top football agent Jorge Mendes, were also searched, according to local media reports.
Around 100 tax inspectors backed by 180 police and a dozen magistrates carried out 76 raids on "clubs, their businesses and directors, as well as lawyers offices and intermediary agents", the prosecutors' office said in a statement.
It added that the raids, codenamed 'Operation Offside', were aimed at "professional football dealings which have taken place since 2015 and which included activities destined to evade paying taxation due to the state".
Media reports said investigators were interested in deals involving the transfer of players, with prosecutors looking for efforts to hide or modify sums involved that could lead to charges including tax fraud and money laundering.
In early February, the Portuguese weekly Sabado reported that a major investigation was underway into several suspicious transfers or contracts, often involving Mendes.
Among the players concerned, the magazine cited current and former Porto players Iker Casillas, Danilo, Radamel Falcao and Jackson Martinez and Benfica stars Raul Jimenez, now with Wolves, and Pizzi.
Suspicion reportedly arose after the 'Football Leaks' revelations, an investigation carried out by European newspapers based on documents obtained by Portuguese hacker Rui Pinto.
Casillas, the former Real Madrid and FC Porto goalkeeper, confirmed his home had been raided.
However, he said he was "absolutely calm" about the investigation and that he was ready to co-operate with the authorities.