Walter Veltroni has stepped down as mayor of Rome so he can stand as the Italian centre-left's lead candidate in general elections.
Walter Veltroni launched his electoral campaign on Sunday
Mr Veltroni leads the newly formed Democratic Party and is set to face a conservative coalition headed by Silvio Berlusconi in the poll on April 13-14.
Elections were called after the centre-left government of Romano Prodi collapsed last month.
Mr Veltroni, 52, has served as a popular mayor of Rome for seven years.
He referred to the period "seven years of wonderful work, one of the most beautiful human experiences that I've ever had".
He is credited with raising the cultural profile of the Italian capital by launching an international film festival, though critics say he did not do enough to resolve practical problems such as traffic congestion.
Italian law forbids mayors of cities of more than 20,000 from standing in legislative elections.
Mr Veltroni comfortably won a US-style primary in October to claim leadership of the party he founded with Mr Prodi.
He launched his campaign on Sunday, calling on Italians to leave behind divisive politics marked by instability and pledging to reduce taxes if elected.
He has said that the Democratic Party will stand alone and not in an alliance with the small parties that have traditionally made up leftist coalitions.
Recent polls suggest that Mr Berlusconi, who has already served two spells as Italy's prime minister, is ahead in the race.