Up to that point, Haslam had been enjoying a strong race, cruising up to the back of the two leaders, only to lose ground again when he ran on at the first corner chicane.
However, his hopes of hunting down Laverty were immediately hampered by Biaggi, the Italian exercising his bike's straight-line pace to cruise back up to second place.
Indeed, although he gave two more chances to Haslam and Melandri to pull away when he made two more identical errors into Variante della Roggia, he was back ahead of them within just a handful of bends.
Even so, while he had no problems getting back up to second place, Biaggi's costly errors had left him 2.4secs behind Laverty, who was revelling from having broken the tow.
Though Biaggi began chasing, Laverty matched him to the chequered flag, the Irishman completing a truly remarkable victory on a day when Biaggi looked to be the man to beat. A repeat of his victory at Monza in 2010, albeit in the Supersport class, the result catapults him up to sixth in the overall standings as a result.
Biaggi consoled himself with the knowledge the key rival and series leader Carlos Checa
could only manage ninth position, bringing his lead down to 30 points.
Despite being sent across the chicane on lap 14 when Melandri elbowed his way through, Haslam recovered to claim his second podium finish of the season in what was arguably BMW's most competitive performance of the year, while Melandri was a fine fourth on his Monza Superbike debut.
Behind the top four, the field was fairly strung out, with Michel Fabrizio
putting in a battling, if lonely, fifth place finish on the Suzuki, up on Jonathan Rea, the front row starter struggling somewhat on the Castrol Honda.
Having jumped out of the blocks, Corser ended up in seventh on the second BMW, well up on Leon Camier, who spent his race locked in battle with Checa, the Briton getting the better of the series leader on the final lap of the race.
Jakub Smrz shadowed his Ducati counterpart in tenth, ahead of Ayrton Badovini, who managed an impressive 11th having been down in last after running across the gravel during the early stages of the race.