Marino stepped down on Thursday over the latest scandal to hit him - after Rome prosecutors opened an investigation into the use of his city hall credit card.

In a letter addressed to Romans, Marino stressed his resignation was not an admission of guilt. Citing Italian law, the former liver transplant surgeon - who has also faced months of accusations that he is too easy on organized criminals - says he could rescind the resignation within 20 days.

"With this gesture I want to draw a line under the pointless and surreal polemics of recent days, which do Rome no good," wrote the centre-left mayor.

Marino agreed to pay back the 20,000 euros ($22,600) of restaurant bills settled on the municipal account hall credit card over the course of his two years. However, it did not satisfy his opponents.

The politician insists he has been targeted after having rooted out corruption, mafia intrusion and far-right infiltration of the city administration.

Police broke up the "Mafia Capitale" network last year, with dozens of politicians and businessmen placed in the frame for allegedly rigging tenders and siphoning off public money. Many of those implicated are due to go on trial next month.

Edged out by allies

However, mounting scandals and investigations have also proved damaging to Marino, who found himself increasingly isolated by allies in his own Democratic Party, led by Prime Minister Matteo Renzi.

The fact that an extravagant funeral for purported mafia boss Vittorio Casamonica was allowed to take place in Rome caused particular embarrassment for the party. In addition were a summer of public transportation break-downs and reports linking dozens of politicians to organized crime.

Although opposition efforts to bring a no-confidence motion were thwarted, Marino was eventually thought to have been handed an ultimatum to quit from his own party.

Critics of the mayor point out that it is not the first time he has been embroiled in an expenses scandal. In 2002 he resigned as the director of a transplant center in Sicily, being accused of submitting the same expenses claim to both it and its US partner the University of Pittsburgh Medical Center.

rc/jr (AFP, AP, dpa)