TOKYO (AP) — Mongolian grand champion Harumafuji has decided to retire from sumo after allegations that he assaulted a lower-ranked wrestler and tarnished the image of Japan's national sport.

"I have caused much trouble to society," Harumafuji said at a packed media conference on Wednesday. "I want to apologize to various people in the sumo federation who have supported me. As a grand champion, I have done something that shouldn't be done."

The news dominated Japanese television talk shows and evening newspapers for weeks as the nation expressed shock at claims against a yokozuna grand champion, whose behavior in sports and society is expected to be exemplary.

Harumafuji was accompanied by his stablemaster Isegahama, who made a tearful apology and said Harumafuji, as a yokozuna — the highest level in sumo — must accept the consequences of actions.

Harumafuji allegedly struck compatriot Takanoiwa with his palms, fists and a karaoke machine remote control at a drinking party in late October, fracturing his skull and causing other injuries.

"As a senior wrestler, I felt it was my responsibility to teach him the proper behavior," Harumafuji said of the encounter in October.

The 33-year-old Harumafuji reportedly became aggravated when Takanoiwa repeatedly checked his mobile phone while the two were conversing.

The Japan Sumo Association, which imposes strict rules on wrestlers, is conducting an investigation, but Harumafuji decided to retire ahead of the probe.

Born Davaanyam Byambadorj, Harumafuji debuted in 2001 and has won the championship nine times, with his most recent victory at the autumn tournament in September. He was promoted to yokozuna in 2012.

Harumafuji's retirement is the latest scandal to rock the sumo world.

In 2011, the JSA decided to cancel the Spring Grand Sumo Tournament after revelations that 14 wrestlers were involved with match-fixing.

In 2010, grand champion Asashoryu, also of Mongolia, announced his retirement following reports that he injured a man while intoxicated.