Among the most famous to come through the club are Zico, a former No. 10 on Brazil's national team; top goal-scorer Adriano, who rose to fame at Inter Milan; and current Real Madrid star Vinicius, Jr., who not long ago was living in the structure destroyed by the fire.
There was no word yet on the cause of the fire and messages to Flamengo officials were not immediately answered. One of the three injured, a 15-year-old boy, suffered severe burns and was transferred to a specialist hospital, reports said.
In addition to the deaths, three people were injured.
A fire broke out in the early hours of Friday at the Brazilian side's Urubu's Nest base that also provides accommodation for some of the club's young players. The ages of the injured were 14, 15 and 16, according to a fire official who spoke on condition of anonymity because of agency rules.
"This is certainly the worst tragedy that this club has gone through in our 123 years with the loss of these 10 people". The fire was reportedly controlled at around 7:20 a.m.
Like many clubs who sign youngsters from across the country, the facility has accommodation for teenagers, although it was unclear exactly how many players were among the dead.
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People wait for information in front of the training center of Rio's soccer club Flamengo, after a deadly fire in Rio de Janeiro, Brazil February 8, 2019.
However, numerous lodgings have a reputation for poor health and hygiene standards.
Vinicius Junior, the teenage Real Madrid forward who trained at the facility before joining for the Spanish giants past year, tweeted his condolences.
"What sad News! Praying for everyone!"
Flamengo is the most popular club in football-crazed Brazil. "Strength, strength, strength", he wrote.
Known by fans as the red-and-black, they won the Copa Libertadores, South America's version of the Champions League, in 1981 and lifted the Intercontinental Cup a few months later. "May the Black and red nation have the strength and faith to overcome this moment". But rival teams across Brazil, including fellow Rio sides Botafogo, Fluminense and Vasco da Gama, tweeted their solidarity.