Former Socialist prime minister Jose Socrates was arrested at Lisbon airport late on Friday as part of an inquiry into tax fraud, corruption and money laundering, the public prosecutor's office said.

Socrates was expected to appear in court on Saturday, local media reported. He is one of four people detained in connection with the case, following a string of police raids involving more than 60 officers.

The prosecutor's office added that the investigation centered on bank operations and money transfers from an unknown source. It stressed, however, that the case was not linked to the money laundering investigation which led to the July arrest of Ricardo Salgado, the former head of crippled Portuguese lender Banco Espitiru Santo (BES).

Socrates led the Socialists to their first majority in parliament in 2005, one year after he was elected leader of the party.

The 57-year-old served as prime minister for six years, during which time he was involved in several controversies, including his decision to approve the construction of a shopping mall on protected land near Lisbon. Questions were also raised over the authenticity of his university degree.

Socrates resigned in 2011 after the Social Democrat opposition rejected his minority government's fourth austerity package in less than a year. Just two weeks later Portugal was forced to request a bailout package from the European Union and International Monetary Fund - a move Socrates had resisted.

Second scandal in a week

Last Sunday, Portuguese Interior Minister Miguel Macedo announced that he was stepping down in the wake of a corruption scandal involving several senior government officials.

The officials were arrested as part of a probe into money laundering and influence-peddling around "golden visas" - a scheme allowing foreign investors who buy property worth 500,000 euros ($620,000) to receive residency rights in Portugal, as well as visa-free travel throughout the European Union's Schengen zone.

The program was launched in 2012 during the country's debt crisis. "Golden visas" have so far have been issued to around 1,500 people, most of them Chinese.

nm/ipj (AFP, dpa, Reuters)