New Zealand boxer and convicted child-killer Soulan Pownceby was beaten in the first round of the 81kg class on the opening day at the Peristeri hall.
Pownceby triggered a moral debate in New Zealand
Pownceby, whose selection caused a furore his homeland, was stopped in the third round by Turkey's Ihsan Tarhan.
The 29-year-old was jailed for four years in 1995 for the manslaughter of his five-month-old daughter.
Meanwhile, Ireland's sole boxer Andy Lee scored a good points win over Alfredo Lopez in the 75kg class.
New Zealand Prime Minister Helen Clarke forced Pownceby to make a public apology before the Games began.
"He's had a pretty rough time and clearly he's disappointed with that performance," said the New Zealand team chief David Currie.
"He's been great since he's been part of the team and I expect he will continue to be."
Meanwhile, Canadian boxer Trevor Stewardson proved his doubters wrong with victory in the 81kg class.
Stewardson required a long legal battle with the Canadian Olympic Committee (COC) to secure his place in Athens.
He and three team-mates thought they had qualified for the Games through a tournament in Rio de Janeiro in April.
But the Canadian boxing authorities and the COC had dropped the Rio event from their qualifying criteria without telling the boxers, deeming it not competitive enough.
The three fighters took the COC to court and there are five Canadian boxers in Athens instead of the initial two.
Another of the trio involved in the legal battle, middleweight Jean Pascal, was outpointed by Cuba's Yordani Despaigne Herrera.