Left-back Kolarov won the Coppa Italia with Lazio in 2009 but left for City a year later, going on to win the Premier League twice at the Etihad Stadium.
His return to Rome was confirmed on Saturday, the Serbia international signing a three-year contract with Lazio's arch-rivals.
And Lazio fans have already reacted negatively, unveiling a banner labelling him "a worm" at a pre-season friendly, and Kolarov has fanned the flames further.
He told Roma TV: "I cannot, and do not want to, deny my past. But now I am with Roma and just as I gave 100 per cent when I was at Lazio I will now give 100 per cent and more for Roma.
"Lazio was a good experience for me, but from today they are my rivals on the pitch."
Kolarov was asked about new Roma sporting director Monchi's influence on the deal and he replied: "The sporting director was very important in this transfer.
"He was very clear and open with me and I was very open with him: we managed to reach an agreement in basically two minutes."