A second-minute header from Ukraine striker Andriy Shevchenko gave Milan the victory, which leaves them with a nine point advantage over Roma and just two games remaining.
But there were ugly scenes as the game was interrupted twice in an incident-packed second half after Roma supporters hurled fireworks and missiles on to the pitch.
First, Milan's Gennaro Gattuso was stunned by a powerful firework that exploded near him after Roma's appeals for a penalty following a handball were controversially turned down.
Then Milan goalkeeper Dida had to receive medical attention after he was twice struck by objects thrown from the crowd.
The victory completed a home and away double over rivals Roma by a Milan side who have lost just one league game during the campaign. They have also earned the most points in a season since three points for a win was introduced in 1994.
Although a draw would almost certainly have led to Carlo Ancelotti's first scudetto as a coach after his Champions League triumph last season, Milan's positive approach showed they wanted to wrap things up in front of a capacity San Siro crowd.
"This was the best way to win the title -- against Roma at the San Siro," said Milan's Dutch midfielder Clarence Seedorf.
Roma coach Fabio Capello, who enjoyed a successful spell as Milan coach in the early 1990's, paid tribute to his old club.
"Congratulations to Milan who have had a truly great season. They have achieved a record points tally but it was their wins over their direct rivals that made the difference," he said.
SUPERB SHEVCHENKO
Milan, who have now beaten their closest challengers Roma four times in league and cup this season, were firmly in control of the game after the best possible start.
Brazilian Kaka produced a pinpoint cross from the right and Shevchenko rose superbly to head past sprawling Roma keeper Ivan Pelizzoli for his 22nd league goal of the season.
Milan should have gone in 2-0 up at the break as just three minutes before the interval Kaka fed an unmarked Shevchenko but the Ukrainian screwed his shot wide from a promising position.
Roma were furious, though, four minutes after the restart when Francesco Totti's free kick struck Shevchenko's raised arm in the Milan wall and their penalty appeals were turned down.
Despite the mood turning sour, with Roma fans also hurling firecrackers into the stands filled with Milan supporters, the home crowd erupted with joy at the final whistle as Ancelotti celebrated a second major trophy in two full seasons.
"It's fantastic. I'm very happy, especially because of our performance today," Ancelotti told Italy's Sky Sport.
"Roma had a great season and it was right that it should come down to this game," said the Milan coach, who twice finished second in the title race in charge of Juventus.
Milan captain Paolo Maldini and defender Alessandro Costacurta set club records with their seventh scudetto.
"I'm very proud of this team and the club, which has won everything and absolutely deserved the scudetto this year," said Maldini. "The scudetto is always special, whether it's your first or your last."
There were surprise results elsewhere in Serie A with relegation-threatened Perugia beating third-placed Juventus 1-0 and lowly Lecce beating Inter Milan 2-1.
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