Francielle converted a free kick in the seventh minute and Renata Costa increased the lead with a header off a corner in the 10th, then Marta scored with a penalty kick in the 73rd, set up Cristiane's goal in the 80th and sealed the victory in the 88th.
Cristiane became the top women's Olympic scorer with her 11th tournament goal, surpassing retired German star Birgit Prinz. Both had 10 goals coming into Thursday's match.
The opening win moved Brazil atop Group E with three points along with Britain, which earlier beat New Zealand 1-0 at the same Millennium Stadium in Cardiff, Wales.
Brazil has been close to winning a significant title several times but never came through with victory despite talented players such as five-time world player of the year Marta. The Brazilians settled for silver in the last two Olympics.
It didn't take long for Brazil to take control of the match against a Cameroon team making its Olympic debut.
With Marta in command, Brazil held possession and created most of the scoring chances, leaving Cameroon with a few opportunities on counterattacks.
Francielle curled a right-footed free kick over the wall for the first goal and three minutes later Renata Costa headed in a Francielle corner from the six-meter line.
"We never know how your team is going to react in a first match like this," Brazil coach Jorge Barcellos said. "But we practiced set pieces a lot in the past few days because we knew they could make a difference, and that's exactly what happened today."
Marta's first goal came from the penalty spot after she had been brought down in the area. She then cleared a defender and sent a perfect through ball for Cristiane to score.
Cristiane returned the favour in the closing minutes, crossing the ball for Marta's easy goal to seal the win.
"The early goals were defensive errors and they demoralized the team," Cameroon coach Carl Enow said.
Cristiane, playing in her third Olympics, came into the match as a second-half substitute to break the Olympic scoring record.
"This is great, all I can think about is the time I spent trying to recover from my recent injury, practicing alone and not knowing if was going to be able to play in the Olympics," she said.
Brazil lost the last two Olympic finals to the United States. It lost to Germany the final of the 2007 World Cup in China, and last year it was eliminated from the World Cup quarterfinals in Germany after allowing a last-minute equalizer to the U.S. in extra time, eventually losing in a penalty shootout.
Brazil's triumphs in women's football are limited to South American championships and gold medals at the 2003 and 2007 Pan American Games. It finished fourth in the Olympics in 1996 and in 2000.
Brazil has a veteran team this time, with some players participating in their fifth Olympic Games.
The total crowd for both matches at the Millennium Stadium was about 30,000.
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