"The result is clear," Alemanno said in announcing he had conceded the election to center-left rival Ignazio Marino, the Italian news agency ANSA reported.
Marino had 60.5 percent of the vote while Alemanno had 39.5 percent in initial polling after voting stations closed.
In the first round of voting, Marino had been 10 points ahead of Alemanno, a member of ex-prime minister Silvio Berlusconi's People of Freedom party.
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