The Confederations Cup semi-final between Brazil and Uruguay scheduled for Wednesday is at risk of being suspended due to a mass rally planned outside the Mineirao Stadium.
Police have warned there will be no way to stop protestors reaching the stadium if the march attracts more than 100,000 people, as expected, reports Xinhua.
"With that number of people, the event is under threat," Minas Gerais state military police chief Marcio Martins Sant'Ana told O Globo.
"It's impossible for a public force or a military force to act against 100,000, 120,000 or 130,000 people."
More than 6,000 police officers will be deployed at the venue to keep protests peaceful. An additional 1,500 military troops have also been placed on standby.
Minas Gerais social defence secretary Romulo Ferraz said security forces would cordon off the stadium's surrounding area, with access granted only to ticket holders.
Earlier, a federal court judge rejected a request from activists to rule the no-protest zone illegal on the grounds it denied freedom of expression.
Protests began in Sao Paulo last week in response to higher public transport fares, but they have turned into a nation-wide rally against corruption.
Many demonstrators have expressed anger at the billions of dollars spent on new football stadiums for next year's World Cup instead of improvements to the country's precarious public services like health, transport and education.
The marches have been marred by violent clashes between protestors and police, who have been forced to fire rubber bullets and tear gas in several cities.
Wednesday's winner is due to play the victor of Thursday's other semi-final between Spain and Italy in the final at Rio's Maracana stadium Sunday.
