Mr Alemanno is a former youth leader of the neo-fascist MSI
A right-wing candidate has won Rome's mayoral poll, taking control of the Italian capital from the left for the first time in 15 years.
Gianni Alemanno won 53.6% of the vote, with his centre-left rival Francesco Rutelli trailing at 46.3%.
Mr Alemanno had been criticised for his neo-fascist roots during his campaign, which stressed tackling violent crime.
His win is another triumph for newly-elected centre-right PM Silvio Berlusconi who won April's elections.
The BBC's Christian Fraser in Rome says the city has long been seen as a citadel of the centre left.
"This long battle has ended with our victory," Mr Alemanno, aged 50, was quoted as saying by the AFP as his lead became evident.
"This wasn't a victory for a party but for the whole of Rome," he said. "The result shows I received broad support and I will take that into account when governing the city."
Mr Alemanno campaigned on the ticket of a tough approach on crime, after a series of attacks blamed on immigrants.
He promised to put more police on the streets of the city and to expel thousands of Roma travellers living in illegal settlements around the city.
The conservative wears a Celtic cross, the symbol of the far right, around his neck.
He is a former youth leader of the neo-fascist Italian Social Movement, the MSI, and is married to Isabella Rauti, the daughter of Pino Rauti, a leading figure on the far right.
Mr Alemanno's victory might also be the final slap in the face for the outgoing centre-left government which collapsed in spectacular style earlier this year, our correspondent says.
Mr Alemanno's rival in the contest was outgoing Deputy PM Francesco Rutelli.
He must carry part of the blame for the failures of Romano Prodi's government - and accordingly has just been punished, our correspondent says.