Workers building Rio de Janeiro's 2016 Olympic Park have fought with security guards but although shots were fired no one was injured in the melee.
Scuffles broke out between guards and construction workers on strike for more pay and better union representation.
The workers closed several busy avenues around the site and trouble ensued.
Officials say they're examining CCTV footage to identify who fired the shots.
The Olympic Park is one of the focal points of Rio's Olympic Games and will host judo, basketball, tennis, handball and cycling events at a host of purpose-built facilities.
The international broadcast centre and the main media base will also be located in the park, which is being built on the site of Rio's former Formula One track.
The consortium responsible for the park, which is slated for delivery in the first half of next year, vowed the strike would not affect delivery dates.
But there is already concern at Rio's slow pace of work.
Construction workers fixing the Joao Havelange stadium that is due to host the track and field events have also downed tools in protest over working conditions and low wages.
Rio won the right to host the summer Olympics in 2009 and will become the first South American city to host the world's biggest multi-sport event.
Brazil will also host football's World Cup this June and July.
