Rome Mayor Gianni Alemanno has quickly conceded defeat in the city's election, acknowledging a big win by center-left challenger Ignazio Marino.

Alemanno told reporters Monday shortly after polls closed that he had congratulated Marino, a transplant surgeon who jumped into politics in 2006. The outgoing mayor called the loss a blow to his center-right party, which is headed by media mogul Silvio Berlusconi, the ex-premier who had campaigned for Alemanno.

With more than half the ballots counted, Marino had garnered about 64 percent of the vote; Alemanno, about 36 percent in the runoff race.

In his laconic style, Alemanno glumly described the loss for Berlusconi's People of Freedom party as "not positive." The party is the main partner in the national coalition government of center-left Premier Enrico Letta.