The champions played for more than an hour with a numerical disadvantage after Wesley Sneijder was sent off but they dominated throughout to earn a richly deserved victory yesterday.
Inter coach Jose Mourinho said that decision showed that the authorities are ganging up on his club.
"Tonight everything was done to try to stop Inter winning but this group is strong and will win the title," he said.
"When I went into the changing rooms (at half-time) I asked (the referee) why he sent off Sneijder and he told me it was because he had applauded him.
"He asked me what I would do if a player ironically applauded one of my decisions. 'No, I replied, I wouldn't have chased him away, we're football men'.
"Hence I understood that this dismissal did not happen just like that."
Milan may still have a game in hand on their neighbours but the psychological effect of such a crushing defeat will make the gap seem all the wider.
Milan coach Leonardo seemed unsure about his side's title chances now.
"I don't know, I think we'll keep going along the path we've taken, I still believe in the way we play," he said.
"We did some incredible things in previous games, we've done much to have got here in second place, two points ahead of AS Roma with a game in hand, we've got a Cup match to come and we still have to play Manchester United in the Champions League."
Milan may have come into the game as the form team in Serie A but Inter were clearly the more pumped up early on.
Sneijder almost fired them in front with a moment of outrageous skill, flicking the ball up and thrashing a 25-yard volley that glanced off the outside of the post.
The Dutch playmaker should then have given the hosts the lead as a ricochet from a Goran Pandev shot fell into his path as he broke into the box but he rammed his shot straight at goalkeeper Dida.
Milan didn't seem to heed those warnings and Inter deservedly took the lead on 10 minutes as Pandev's ball over the top saw Diego Milito get behind Ignazio Abate in the inside left channel and shoot low across Dida and into the far corner.
Inter were rampant and another break saw Pandev feed Milito on the inside right but Dida blocked his near post shot.
Inter were dominant and Sneijder was running the show but the referee changed that before the half hour was even up.
Inter centre-back Lucio burst forward into midfield and went down rather easily under a challenge but seeing that he still had the ball he got up and went to carry on, only for the referee to stop the game and book him for diving.
Inter were incensed and Sneijder's sarcastic gesture saw him sent off.
Sneijder was beside himself and needed to be dragged from the field by two team-mates.
Inter knuckled down, though, with stalwarts Javier Zanetti and Esteban Cambiasso looking to control the midfield.
Milan were disjointed and struggling to create anything, with their best chance coming from a heavily deflected Andrea Pirlo free-kick that wrong-footed Julio Cesar but looped up and landed on the roof of the net.
Milan came flying out in the second half and Julio Cesar saved a close range Clarence Seedorf header from a David Beckham corner before Ronaldinho hit a swivelling volley just wide.
Inter were still dangerous on the counter and Milito almost made Milan pay with one break but the visitors were gradually imposing themselves with Beckham crosses from the right causing particular bother as Marco Borriello headed one just over.
But then from another brilliant break Milito slipped in Pandev who chipped Dida only to see the ball come back off the post and into the Brazilian's grateful arms.
But on 65 minutes Giuseppe Favalli brought down Maicon on the edge of the box and Pandev stepped up to curl home a free-kick Dida could do nothing about.
Earlier in the day Napoli won 2-0 at Livorno to move to within a point of third-placed AS Roma while Palermo's 3-0 defeat of Fiorentina moved them up to fifth.