"If this happens again, we have to question whether we made the wrong decision awarding the hosting rights," Blatter told German press agency DPA.
FIFA has verified Blatter's comments were accurate.
FIFA has acknowledged it spoke with Brazilian officials after the Confederations Cup final, which closed with police and soldiers firing tear gas, shock bombs and rubber bullets to keep thousands of protesters away from the Maracana stadium 200 meters (yards) away. Brazil defeated Spain 3-0 in the final with tear gas wafting through the stadium during the first half.
Blatter said he would discuss the protests when he meets Brazilian President Dilma Rousseff in September.
"We didn't do a political debriefing, but we did emphasize the fact of this social unrest being there for the entire duration of the Confederations Cup," he said. "The government is now aware that next year the World Cup shouldn't be disturbed.
"To me, these protests were like alarm bells for the government, the senate, the parliament. They should work on it so that this is not going to happen again. Though protests, if peaceful, are part of democracy and therefore have to be accepted ... we are convinced the government, and especially the president, will find the words and the actions to prevent a repeat. They have a year to do so."
In its statement, the sports ministry also emphasized the freedom to protest.
"As for the demonstrations, Brazil is a democratic country that guarantees its citizens full freedom of expression."
In April, FIFA's top administrator Jerome Valcke suggested "less democracy is sometimes better for hosting a World Cup."
Small street protests have continued in Brazil since the Confederations Cup, and security officials are bracing for more trouble with the arrival Monday in Rio de Janeiro of Pope Francis for the Catholic Church's youth festival.
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