LISBON (Reuters) - Portuguese police have arrested former Socialist prime minister Jose Socrates and three other people in an investigation of suspected tax fraud, corruption and money-laundering, the Prosecutor General's office said on Saturday.

It said in a statement that Socrates, who was detained on Friday, would be brought before a criminal judge for questioning on Saturday regarding an investigation of suspicious banking operations and money transfers.

Portugal's official news agency Lusa said police arrested Socrates at Lisbon airport as he arrived from abroad.

Socrates resigned as prime minister in the middle of his second four-year term in 2011 as an escalating debt crisis forced him to request an international bailout that imposed painful austerity on Portugal.

A snap election that year brought to power the current center-right ruling coalition, which introduced the unpopular austerity measures.

After stepping down, Socrates left for Paris, where he attended a university course. He returned in 2013 to become a regular commentator on RTP state television.

The center-left Socialist party leads in opinion polls ahead of next year's general election.

(Reporting by Andrei Khalip; editing by Andrew Roche)