NAGOYA, Japan — Ahmed Fathi, a defensive midfielder, ran for his life when he saw thousands of Egyptian opposition supporters streaming toward him on the field. His team, Al Ahly of Cairo, had just lost a local league game in February to Al Masry in the city of Port Said.

Al Ahly does not lose often. It is the biggest and most successful soccer club in Egypt, and it claims to have tens of millions of fans worldwide. But the Masry supporters were not celebrating their victory. Something had gone terribly wrong.

“The fans were coming, sprinting after the match,” Fathi, 28, recalled last week. “I knew they hated me and all the players. All the players ran. I didn’t know what was happening outside. But something was happening outside. After this they killed the boys. Not the men, the boys.”

As Fathi and his teammates took refuge from the Masry supporters in a changing room, one of the darkest incidents in soccer history was unfolding in the nearby bleachers.