Ronaldinho is under pressure in Brazil after Flamengo's elimination in the group stage of the Copa Libertadores.
The Lance! sports daily reported on Friday that Ronaldinho's bodyguards had to draw guns to fend off protesting supporters as he tried to leave the stadium after Thursday's elimination and the team's biggest fan group published an open letter accusing the two-time Fifa world player of the year of disrespecting Brazil's most popular club.
Ronaldinho set up two goals in Flamengo's 3-0 win over Lanus of Argentina but the victory was not enough to advance the Brazilian club to the second round of Latin America's premier club competition.
It was reported that two bodyguards drew their guns and pointed them into the air to make way for Ronaldinho's exit from the Engenhao stadium.
About 25 fans were outside the ground yelling "shame" and other words to complain about the team's poor performances in this year's competition. They were blocking Ronaldinho's car, prompting the bodyguards to exit their vehicle and threaten the supporters. The fans had already thrown rocks and beer cans at the team's bus.
The fan group Raca Rubro-Negra published a letter on its website criticising Ronaldinho, claiming he has not reproduced the level of play he displayed when with Barcelona or even with Milan later in his career.
"We are not here to question the club's policy, only the performance by Ronaldinho, which is not pleasing anyone who understands football," the letter read. "We are not satisfied with this player who is disrespecting the club and is not giving his best."
The fan group said Ronaldinho's skills "apparently were left in the past" and accused the player of being "unprofessional".
"The player with the highest salary in the squad, who should serve as an example for the youngsters who are beginning their careers, only shows a lack of respect to the colours of Flamengo."
The group was especially upset with an alleged barbecue they say Ronaldinho organised "for friends at his mansion" in Rio after the team's 3-2 defeat by Emelec in Ecuador a week ago.
Emelec qualified along with Lanus in Group 2 of the Copa Libertadores thanks to an injury-time victory over Olimpia on Thursday. Flamengo would have advanced only if that match had been a draw.
The disappointing elimination and the criticism by fans prompted reports that Ronaldinho might leave the club. But the player's brother and agent, Roberto de Assis, said on Friday that Ronaldinho intends to remain with Flamengo at least until his contract expires in 2014.