Pele, speaking at a press conference in a Mexico City suburb, said he had no doubt that demonstrations that have reemerged in recent weeks would affect the tournament if they continue.
"We already know that 25 percent of foreigners who were going to Brazil are worried by the protest movement, and I think they have cancelled," said the 73-year-old honorary ambassador to the tournament.
"This is a great loss for the country," said Pele, who won his third World Cup in Mexico in 1970.
Some protests have turned violent, but they are smaller than the demonstrations that brought one million people to the streets last year during the Confederations Cup, a warm-up to the World Cup.
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Pele, who was criticized last year for urging Brazilians to leave the streets and focus on football, said he agreed with the protesters' grievances, like the need for more schools and hospitals.
"We have nothing to do with corrupt politicians and thieves. It is not our fault," the former Santos great said, arguing that players should not be lumped together with "the thieves who stole to make stadiums."
Pele also voiced dismay that some of the 12 World Cup stadiums have yet to be finished 24 days from the first game.
Turning to the national team's chances of winning its sixth World Cup, Pele said he saw weaknesses in the attacking side and voiced concerns about putting too much pressure on young star forward Neymar.