Television pictures showed chaotic scenes in front of the Mangueirao stadium, in the northern city of Belem, where an estimated 75,000 people turned up hoping for a unique opportunity to see the world champions in the flesh.
Desperate fans, including children, were seen running, falling over each other and being trampled on amid a background of screaming and wailing sirens.
Riot police were brought in and used pepper spray to try and control the situation.
The stadium, which holds around 45,000, was full one hour before the practice session, causing angry fans outside to jump over the fences or force open the gates.
Jaime Bastos, the doctor responsible for the stadium's first aid post, said he had lost count of the number of people he had treated.
"When everyone comes in at once, you can't note down anything," Bastos said.
The Globonews cable television station put the injury toll at around 100, while other media said dozens had been hurt.
A police spokesman told Reuters that they had expected around 35,000 for the practice but that 65,000 had got inside the stadium and around 10,000
Entry was free but fans were asked to bring their own food.
Brazilian media initially reported that a 10-year-old girl had been killed in the crush.
They later said local authorities had denied the incident happened at the stadium and was the result of a road accident nearby.
Brazil regularly take their home matches to the provinces, playing in cities which would otherwise never see top-level international football.
Their practice sessions are popular with ordinary fans who cannot afford tickets for the games themselves and want a glimpse of players such as Kaka, Ronaldo, Adriano, Roberto Carlos, Ronaldinho and Robinho.
Brazil's top players are all based in Europe, many having left in their teens and early 20s after playing only a couple of seasons in their homeland.
Inside the stadium, where the crowd and players were apparently unaware of the problems, there was a festive atmosphere reminiscent of a pop concert, with high-pitched screaming when players such as Kaka and Robinho touched the ball.
With one round of fixtures remaining in South American qualifying, Brazil have already secured a spot at next year's finals in Germany, while Venezuela are out of contention.