Cameroon's Hassan N'Dam N'jikam, a former WBO interim middleweight champion who is now fighting as a light heavyweight, lost to Brazilian amateur Michel Borges in a unanimous decision that delighted a raucous home crowd.
Earlier on Saturday, Italy's Carmine Tommasone made boxing history when he became the first fully professional fighter to compete, winning his opening bout against a Mexican amateur.
The two professionals and a Thai fighter who has yet to make his Olympic debut are competing after AIBA, the sport's governing body, made a constitutional change in June that ended 112 years of amateur exclusivity at the Games.
For 32-year-old N'jikam, the change to Olympic rules meant a struggle to adapt. Compared with professional boxing, which can feature 10 rounds or more, Olympic bouts are compressed to only three.
As a professional, "I take time in the first round to know what he's going to do," he said, admitting that the cautious approach led to his losing crucial early momentum.
Still, he does not regret competing.
"I am not happy but I (had) fun," he said.
Tommasone, meanwhile, revelled in reaching the Games and winning his first fight.
"The Olympics is the top dream of any athlete," he said, his face still red from a gritty bout in which he struggled to penetrate the guard of a taller opponent, Lindolfo Delgado.
In another shift, this Olympics marks the first time since 1980 that fighters are competing without headgear.
The boxing began in mid-morning on the first full day of competition at the Games after a colourful opening ceremony at Rio's Maracana stadium the previous night.
The 9000-seat boxing venue, during morning and afternoon fighting, began with few spectators, but was approaching a full house by the time light heavyweights entered the ring at the end of each session.
Enthusiastic fans, especially yellow-clad Brazilians who made the arena shake as they stomped on metal stands, cheered the bouts featuring fighters from countries including Britain, Spain, China, Uzbekistan and Poland.
The winners of the opening bouts will move forward into the round of 16.
(Reporting by Paulo Prada; Editing by Toby Davis and Bill Rigby)
