Officers in Portugal's northern city of Porto have detained the leader of the Jewish community there, saying he issued the document allowing Russian billionaire Roman Abramovich to get Portuguese citizenship last year, authorities said on Friday.

Officers from the criminal investigation agency PJ detained rabbi Daniel Litvak as part of an ongoing inquiry by public prosecutors into the granting of citizenship to Chelsea soccer club owner Abramovich, Publico newspaper said earlier.

In a statement, Porto's Jewish community denied any wrongdoing and said it is was the target of a “smear campaign”.

It said that rabbi Litvak oversaw the department that certifies Portuguese nationality on the basis of criteria that "have been accepted by successive governments”.

Abramovich was granted citizenship in April 2021 based on a law offering naturalisation to descendants of Sephardic Jews who were expelled from the Iberian peninsula during the Mediaeval Inquisition.

There is little known history of Sephardic Jews in Russia, although Abramovich is a common surname of Ashkenazi Jewish origin.

Applicants' genealogies are vetted by experts at one of Portugal's Jewish centres in Lisbon or Porto. The Porto centre, where Litvak is the rabbi, was responsible for Abramovich's process.

Public prosecutors opened an inquiry in January.. On Thursday, a Portuguese government source told Reuters Abramovich's citizenship could be stripped depending on the outcome of the inquiry.

In a joint statement, PJ and the public prosecutors said a suspect had been detained on Thursday but did not name the rabbi, adding the suspect would appear before a judge.

It said its officers raided homes, a lawyer's office and others spaces on Friday as part of the investigation into crimes such as money laundering, corruption and falsification of documents. Evidence was collected and would be analysed, it added.

Litvak was preparing to travel to Israel when he was detained, Publico said.

Porto’s Jewish community said public prosecutors were also investigating the certification of Portuguese nationality granted by Lisbon’s Jewish community to Franco-Israeli billionaire Patrick Drahi, the founder of Altice telecom group.

Altice was not immediately available for comment.