Iker Casillas has said he is "absolutely calm" after his home was among those targeted in a series of raids on football clubs, directors, agents and players by Portuguese authorities investigating alleged tax evasion and money laundering.

Police officers and tax inspectors searched 76 locations across Portugal early on Wednesday -- including the offices of the country's biggest clubs Benfica, FC Porto and Sporting Club -- in an investigation they are calling "Operation Offside."

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In a statement, they said they are looking into "activities aimed at avoiding the payment of taxes owed to the Portuguese state" in transfers and other football-related business since 2015.

"This morning prosecutors came to my home in Porto, as they did to 76 sporting organisations, players and clubs, to ask me for documentation," Casillas posted on Twitter on Wednesday.

"I have put myself at their disposal. Transparency is one of my principles."

In a further statement, his campaign team said Casillas wants "transparency to reach every corner of football" and that he has "collaborated with [the authorities] at all times and provided all the documentation they have requested."

FC Porto also released a statement, which read: "FC Porto and its chairman of the board of directors confirm that they were the target of searches carried out by the Central Court of Criminal Investigation. As always, the club will cooperate."

Former Spain and Real Madrid goalkeeper Casillas -- who was in the Spanish capital when investigators arrived at his home in Porto -- has said he has "full confidence" in the Portuguese authorities.

The 38-year-old joined Porto from Madrid in the summer of 2015 and had maintained a role at the club as he continued his recovery from a heart attack suffered in training in May last year.

Casillas confirmed last month that he intends to stand for president of the Spanish football federation, the RFEF, against the incumbent Luis Rubiales when elections are held at a yet-to-be-determined date later this year.