The Benfica stadium in Lisbon and the premises of three other Portuguese Premier League clubs were searched on Monday in an inquiry into "sporting corruption," a judicial source told AFP.

The prosecutor in Porto said in a statement that it had conducted 24 searches, four at sports clubs, in Lisbon and Setubal in the south, and several in towns in the north as part of an inquiry into "crimes of active and passive bribery, influence peddling and obtaining or offering an undue advantage."

According to an AFP source, the clubs were Benfica, Vitoria Setubal, Desportivo Aves and Pacos de Ferreira.

The investigators suspect Benfica of offering incentives to players at three other clubs so that they would try harder to beat Porto, who, nevertheless, ended up beating Benfica to the Portuguese title by seven points.

Benfica released a statement saying that the inquiry was sparked "by a an anonymous accusation made in Porto" and saying Benfica had always acted "with a scrupulous respect for the law."

Benfica has been embroiled in a series of recent corruption cases in recent months.

In March, a club official, Paulo Goncalves, and four other people were charged over alleged bribes to access information on ongoing investigations by the Ministry of Justice.

In January, club president Luis Filipe Vieira and his number two Fernando Tavares were implicated in a case involving trying to exert influence on a Lisbon Court of Appeals judge.

In October, Goncalves was indicted in another investigation, dubbed the "e-mail business", into a system for bribing referees to help Benfica.