Simoncelli, 24, suffered chest, head and neck injuries after he lost control of his Honda and swerved across the track, into the path of other riders.
The motorcyclist had been tipped by some as a future world champion.
Such were the numbers expected at the funeral, that big screen TVs were erected in two of its squares.
Flowers and messages were left in the town centre, many bearing the number 58, which he wore on his helmet, or the abbreviation “SIC”, which was used for his name onscreen during races.
The hashtag #CiaoMarco (Bye, Marco) was trending on Twitter in Italy.
Young mourner Ilaria Sandri was among those who had come to the north-eastern town.
“We are here because we’ve been following Marco since the very beginning, and aside from being a great rider he was a true person, fame did not change him at all,” she told Reuters news agency.
Another fan, Rossella Fornasari, said: “We have come from Parma, we got up at 04:30 to be here.
“We felt we had to come because we had the honour, not the luck, to know him. He was a great person, a genuine person, on and off the track.”
Simoncelli died after being struck by fellow riders Colin Edwards and Valentino Rossi. Mr Rossi, who was a friend of Simoncelli’s, attended Thursday’s funeral.
“Everything happened so fast,” Honda Gresini team chief Fausto Gresini told Italian newspaper Corriere dello Sport on Tuesday.
“I’m lost for words. I know our job is dangerous, that risk is part of the game, but you always hope nothing happens…
“The crash was caused by a sequence of incredibly negative circumstances, the bike that moved towards the inside of the turn instead of the outside, being run over on the widest track of the season.”