Romania's Prime Minister Victor Ponta was questioned by prosecutors on Monday as part of a corruption probe that has sparked calls for him to resign.

Ponta hobbled on crutches through a crowd of reporters into the headquarters of the anti-corruption agency (DNA) without making any comment.

After returning to work on Thursday following a month-long stay in Turkey for a knee operation, Ponta said on Sunday he was stepping down as head of the Social Democrat party while he fought the corruption allegations, but said he would stay on as premier.

In June the DNA launched a probe against him for fraud, tax evasion and money laundering dating to 2007-11, before he became prime minister in 2012.

Separately, prosecutors suspect he is guilty of conflict of interest as premier. But that probe was stymied when parliament, where Ponta's party has a comfortable majority, refused last month to lift his immunity from prosecution.   

The centre-left prime minister's legal troubles prompted a fresh crisis in the European Union's second poorest nation, where President Klaus Iohannis and the opposition has called on Ponta to step down.

Ponta has firmly denied the allegations and has promised to cooperate fully with prosecutors.

The money laundering and tax evasion allegations relate to his activities as a lawyer.

Ponta is accused of receiving the equivalent of around 55,000 euros ($61,000) from Dan Sova, a political ally and member of parliament suspected by prosecutors of abuse of power but who also enjoys immunity.

This probe is the latest in a string of enquiries by the DNA, an anti-corruption agency within Romania's prosecutor's office, that have cost several prominent Romanians their jobs in recent months.