A small group of protesters was stopped by heavily armed security near the site of the opening ceremony for the Rio Olympics.
As they tried to get closer, security pushed them back, which set off the clashes.
The armed security officials reportedly fired tear gas to keep the protestors from reaching the venue.
The protesters got within about 1.5 miles of the Maracana stadium.
Television footage showed two protesters setting fire to a yellow shirt with the logo “Rio 2016”.
The shirt was worn by a staff working at the Olympics.
It was not clear how many protesters were detained, though footage showed at least one young man being placed into the back of a police van.
There have been consistent small protests in Brazil in the run-up to the Olympics, much of them aimed at the torch relay as it circled the country of 200 million.
Brazil is in the midst of its deepest recession since the 1930s.
Unemployment is above 10 percent, and the local currency has tumbled in value against the dollar in the past year.
[Contributions by Associated Press]
