Portugal's former Socialist prime minister Jose Socrates was arrested Friday as part of an inquiry into tax fraud, corruption and money laundering, the public prosecutor's office announced.

Socrates is one of four people who have been held in recent days, three of whom appeared before a judge on Friday, the prosecutor said in a statement.

More than 60 police, customs officers and justice officials carried out raids at several unspecified locations ahead of the arrests.

Socrates, 57, was arrested in the afternoon as he arrived at Lisbon airport and will appear in court on Saturday, according to Portuguese media.

The investigation centres on bank operations and money transfers from an unknown source, the prosecutor's office said.

But it made clear that the case under which Socrates was held is not linked to the arrest in July of Ricardo Saldago, former head of stricken Portuguese lender Banco Espirito Santo (BES).

The news of Socrates' arrest comes hard on the heels of another scandal which cost Portugal's Interior Minister Miguel Macedo his job on Sunday.

Macedo resigned after several senior government officials were arrested as part of a probe into money laundering and influence peddling around so-called "golden visas".

Under the "golden visa" scheme, foreign investors buying property worth 500,000 euros ($620,000) or more and keeping it for at least five years receive residency rights in Portugal and, more importantly, visa-free travel throughout the European Union's Schengen zone.

Portugal rolled out the programme in 2012 while grappling with a debt crisis.

Deputy Prime Minister Paulo Portas said Thursday it would be a mistake to stop offering wealthy foreign investors the "golden visas".

Socialist Socrates, who has described himself as a "ferocious animal" when it comes to campaigning, resigned in 2011 after the parliamentary opposition, led by the Social Democrats, rejected his minority government's fourth austerity package in just under a year.

Two weeks later Portugal was forced to request a bailout package from the European Union and the International Monetary Fund because of its inability to meet its refinancing obligations, a move Socrates had resisted for months.

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

The former premier faced several other controversies which tarnished his image during his six years in power.

He has faced repeated questions over clearance he gave for the construction of a shopping mall on protected land near Lisbon when he was environment minister, just days before a 2002 general election in which the Socialists lost power.

An investigation into the so-called "Freeport" case, named after the shopping mall, was finally shelved in July 2010.

There have been questions, too, over the authenticity of his university degree.

After he left office, Socrates studied at the institute of policy studies in Paris before returning to Portugal to start a new career as a commentator on RTP public television.