Tifosi unfurled banners saying "Kaka is Priceless" and "Hands Off Kaka." Others waved 50-euro notes at AC Milan President Adriano Galliani.
Milan is considering a $150 million offer for Kaka from Manchester City.
Most within the club, including Beckham, hope the deal doesn't go through, but Silvio Berlusconi, the Italian prime minister and former president of AC Milan, is believed to support an offer too good to refuse from City's new owner, Sheikh Mansour al-Nahyan.
One banner read "Berlusconi, Interista"-- referring to city rival and Serie A leader Inter Milan.
City executive Garry Cook, Sheikh adviser Simon Pearce and Kaka's father, Bosco Leite, were expected to meet in Milan some time later on Monday in an attempt to iron out a deal.
Berlusconi's support for a deal is believed to stem from his concern that the economic downturn in going to have a huge impact on the transfer market driven by England's top clubs, many of whom have credit issues.
Meanwhile, Beckham had an average performance in his debut at the Giuseppe Meazza Stadium.
"He was tidy, very disciplined," Milan coach Carlo Ancelotti said. "He can't yet play for 90 minutes, he felt Fiorentina's pressure a bit in the second half. Apart from the tactical discipline, what I like about him above all is that he has a very low error rate."