Alan Fisher, Al Jazeera's correspondent in Kabul, said there was speculation that Afghan security guards could have fired in the air to disperse the crowd. Afghan officials, however, say they did not fire.
"Dear brothers and sisters, with respect to everybody, we are finishing our ceremony here," Karzai said as he left the stage.
An estimated 25,000 people, including cabinet members and foreign diplomats had gathered at the stadium which is near the presidential palace.
The ceremony was held to mark the sixth anniversary of the death of Ahmad Shah Masood, a military leader of an anti-Taliban alliance, killed by a suspected al-Qaeda suicide bomber two days before the September 11 attacks in the United States.
Kabul's already tight security is usually heightened for major events, amid fears of attacks by Taliban fighters.