Justice Roberto Barroso, a Yale graduate and constitutional law professor, said 50 years of war against drugs had failed miserably, clogging jails with small-time dealers and fueling a violent gang battle for control of the lucrative trade.

"Unlike the United States and Europe where the problem lies in the impact drugs have on consumers, in Brazil the problem lies in the power drug traffickers have over poor communities," Barroso told Reuters in the court's modern glass building in Brasilia.

"I can assure you it is only a matter of time. Either we legalize marijuana now or we do it in the future after we have spent billions and incarcerated thousands."

The rare appeal from a top judge in the deeply conservative country reflects rising fears about the violence plaguing Brazil's overcrowded prisons and city slums.