Former Portuguese Prime Minister Jose Socrates has arrived back at a Lisbon courthouse for further questioning about suspected corruption, money-laundering and tax fraud.

Socrates arrived at the court in a police car on Monday after spending a third straight night in jail.

A judge is conducting a preliminary hearing, after which he must rule whether Socrates should be released or kept in custody pending a trial.

Socrates, Portugal's centre-left Socialist prime minister from 2005 until 2011, was detained by police late on Friday.

Authorities said he and three others were suspected of crimes, but declined to provide details.

Portugal's judicial secrecy law forbids the disclosure of information in ongoing investigations.

Battling corruption

Criminal judge Carlos Alexandre previously interrogated Socrates, 57, on Saturday and Sunday as part of the ongoing inquiry.

On Sunday, Socrates' lawyer, Joao Araujo, told reporters the former prime minister was "in good spirits".

The crime of corruption carries a prison term of up to eight years.

The detention, the first involving a former premier in Portugal under democracy, followed arrests of other high-ranking officials in separate cases over the past few months, as prosecutors intensify a fight against corruption in a country notorious for its slow justice system.

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, which imposed painful austerity on Portugal.

Source: Agencies