
The Milanese champions now have 14 points from six games, while Roma were held on 11 to be leapfrogged by Fiorentina, who moved on to 12 points by crushing 10-man Livorno 3-0 in a late Tuscany derby.
Roma could also be leapfrogged by Juventus, Palermo and promoted Napoli, all on 10 points before respective games they are to play Sunday against Torino, Empoli and Genoa.
In Rome, a bold and fast start proved fruitless for Roma as Mancini squandered a chance after one minute, firing a rebound on Julio Cesar after the Brazilian goalkeeper had managed to save a nasty free kick from Francesco Totti.
Roma held the upper hand in the first half hour but failed to seriously threaten while Inter only managed two harmless drives through Zlatan Ibrahimovic and Esteban Cambiasso.
The game's turning point came on 29 minutes when Ibrahimovic converted a spot kick and Roma had their French midfielder Ludovic Giuly sent off for saving with his hands a goalbound header from the towering Swede.
Brazilian defender Maxwell pounced on an ill-conceived corner kick from Totti and sent away Cesar, whose drive was blocked by the onrushing Alexander Doni and became an inviting assist for Ibra.
The Swedish star took the spot kick and notched his seventh goal to seize the lead in the scorers table, one goal above Juve's David Trezeguet, but had to leave the pitch with a foot injury early in the second half.
A fast phase after the break yielded Roma a 53rd-minute equalizer through a low drive from Simone Perrotta, but Inter reacted promptly with substitute Julio Cruz hitting the post and Hernan Crespo, himself a substitute for Olivier Dacourt, tapping in after a fast Cambiasso shot.
Cruz made the score 3-1 on the hour firing home from the distance, and eight minutes later defender Ramiro Cordoba nodded in Inter's fourth goal off a Luis Figo cross.
In Livorno, Fiorentina closed the first half one up through a goal by Argentinian debutant Pablo Osvaldo, who scored again on 65 minutes after the dismissal of midfielder Massimo Loviso for a from-behind foul.
Alberto Santana, also from Argentina, scored Fiorentina's third goal for a win the Florence side last clinched at Livorno in 1935.
DPA