RIO DE JANEIRO (Reuters) - Another professional fighter landed a victory at the Rio Games on Sunday, a second day of Olympic boxing that also featured one lightweight arguing outside the ring that organizers had conspired against him.

Amnat Ruenroeng, a 36-year old former IBF flyweight champion from Thailand, defeated Argentine rival Ignacio Perrin to become the second pro, along with Italian lightweight Carmine Tommasone, to reach the second round.

A third professional, Cameroon's Hassan N'Dam N'jikam, lost on Saturday after a bout in which the boxer, accustomed to longer fights, said he struggled to adapt to the three-round format of the Olympics.

The Thai fighter, by contrast, said on Sunday that he knew he had to strike early to win.

"I needed to get some scores in the first round because there are only three," said Ruenroeng, who is now fighting as a lightweight.

Rio 2016 is the first time in Olympic history that fully professional fighters are allowed to compete, following a change in June by AIBA, the organization that oversees boxing at the Games.

The association, not immune to the fractious organizational squabbles that plague the sport, on Sunday came under heavy criticism by Teofimo Lopez, an American-born lightweight who is boxing for Honduras.

Losing after a spirited bout with a French rival, Lopez said that the judges had ruled against him because he had criticized a convoluted qualifying process that kept him from competing as an American.

"AIBA stole my dream," he said, shortly after an upbraiding by his father for not following his coach's instructions to fight more aggressively and ensure a win.

Looking on, an AIBA spokesman dismissed the claim.

"We understand his disappointment," said Nicolas Jomard, the AIBA official. "Each boxer is free to say what he wants but any accusations need to be based on something tangible."

(Reporting by Paulo Prada; Editing by Frank Pingue)