During the protest, another man in plainclothes who identifies himself as a police officer also pulls a pistol and fires two shots into the air near the stadium, where Argentina played Bosnia-Herzegovina.
Government spokesman Pedro Dantas said there would be no official comment until the video could be viewed, and that there had been no reports of people having suffered gunshot injuries.
The incident took place around the beginning of the group match between Argentina and Bosnia-Herzegovina - the first World Cup finals game played in the iconic Maracana stadium since 1950.
Earlier, officers had fired stun grenades and tear gas to block a march by about 200 protesters heading towards the stadium.
A stand-off ensued, with banner-brandishing demonstrators massing near a police line guarding the route to Maracana.
Outnumbered both by security forces and journalists, the protesters chanted "Fifa, go back to Switzerland," referring to international soccer's governing organisation.
The protesters are angry over the lavish public spending on stadiums for the World Cup while conditions in Brazil's schools and hospitals remain woeful.
Another protest occurred in the capital, Brasilia, but drew only a handful of participants, while a small protest also was held in Porto Alegre.
Mass protests broke out across Brazil during last year's Confederations Cup soccer tournament. At that time, more than one million Brazilians took to the streets in a single day in the largest demonstrations this South American nation had seen in a generation.
Although anger about World Cup spending remains widespread, protests that have been staged since the tournament began last week have failed to draw strong public support, generally attracting only a few hundred protesters.
A heavy presence by security forces outside Brazil's 12 World Cup venues has also helped keep demonstrations under control.