The incident occurred around 8 p.m. during an annual music festival sponsored by WHFS-FM. A 20-year-old man had a heart attack and was taken to a local hospital, District of Columbia fire spokesman Alan Etter said. The man’s condition was unavailable. Witnesses said people in the crowd continued dancing, unaware that the man had fallen.

“You had a bunch of people dancing around. It became very aggressive,” Etter said.

Etter said Eminem was urging the crowd to move back when the stampede started. The rapper resumed his performance after a brief delay, while police calmed the crowd.

Four other victims were taken to area hospitals with non-life-threatening injuries, and several people were treated at the scene for bruises and broken bones.

The mosh pit was moderately crowded, Etter said. The concert, a predominantly alternative music event, was not a sellout.

“There were fights and a lot of pushing and shoving,” said Brent Turner, 17, of Damascus, Md., who said he saw the disturbance.

Etter said the crowd was not unruly, although there was a significant police presence, mostly for crowd and traffic control. Etter could not estimate how many police officers were there. No arrests were reported as a result of the stampede.

Other groups performing Saturday included Sum 41 and N.E.R.D. The two-day festival was to continue Sunday as planned.