KABUL, Afghanistan, March 4 — American troops opened fire on a highway filled with civilian cars and bystanders on Sunday, American and Afghan officials said, in an incident that the Americans said left 16 civilians dead and 24 wounded after a suicide car bombing in eastern Afghanistan. One American was also wounded.

The shooting sparked demonstrations, with local people blocking the highway, the main road east from the town of Jalalabad to the border with Pakistan. And there were differences in some of the accounts of the incident, with the Americans saying that the civilians were caught in cross-fire between the troops and militants, and Afghan witnesses and some authorities blaming the Americans for indiscriminately shooting at civilian vehicles in anger after the explosion.

The United States military said the unit came under fire after a suicide bomber detonated his explosive-laden car near their convoy “as part of a complex ambush involving enemy small-arms fire from several directions.”

Members of the unit, on patrol near Jalalabad airfield, returned fire, and the civilians were killed and wounded in the cross-fire during the battle, according to a statement from the military press office at Bagram Air Base, 40 miles north of Kabul.

“We regret the death of innocent Afghan citizens as a result of the Taliban extremists’ cowardly act,” Lt. Col. David Accetta, a military spokesman, said in the statement. “Once again the terrorists demonstrated their blatant disregard for human life by attacking coalition forces in a populated area, knowing full well that innocent Afghans would be killed and wounded in the attack.”

Yet some of the wounded interviewed in the hospital by news agencies said the only shooting came from the American troops. A hospital official, who asked not to be identified, said all the wounded were suffering from bullet wounds and not shrapnel from the bomb explosion.

Hundreds of Afghans blocked the road and threw rocks at police officers in protest afterward, with some demonstrators shouting “Death to America! Death to Karzai!” a reference to President Hamid Karzai, The Associated Press reported.

The shooting will be a setback for American forces in Afghanistan, who have been working to contain the continuing insurgent attacks, in particular roadside bombs and suicide attacks, and win the support of the people with reconstruction and development projects. Deadly riots shook Kabul last May after American troops were involved in a fatal car crash and then opened fire on the crowd.

Among the dead on Sunday morning were a woman and two children in their early teens, said Dr. Ajmal Pardez, the provincial director of health, speaking by telephone from the Jalalabad city hospital. He said the hospital received 10 dead and 25 wounded people from the incident, with four people in critical condition.

After the suicide attack, the Americans treated every car and person along the highway as a potential attacker, though none of the people showed hostile intent, Muhammad Khan Katawazi, the district chief of Shinwar, told The A.P.

“They were firing everywhere, and they even opened fire on 14 to 15 vehicles passing on the highway,” said Tur Gul, 38, who was standing on the roadside by a gas station and was shot twice in the right hand. “They opened fire on everybody, the ones inside the vehicles and the ones on foot.”

Some of the wounded interviewed by The A.P. said the soldiers opened fire indiscriminately on passing cars and pedestrians on the busy main road.

“When we parked our vehicle, when they passed us, they opened fire on our vehicle,” said 15-year-old Mohammad Ishaq, who was hit by two bullets, in the left arm and right ear. “It was a convoy of three American Humvees. All three Humvees were firing around.”

In other fighting, two British soldiers were killed Saturday in southern Afghanistan, the British Defense Ministry said Sunday. The men were involved in heavy fighting that has raged for three days in the town of Sangin, said Col. Tom Collins, a NATO spokesman in Kabul. Townspeople have fled the town and abandoned their shops as Taliban insurgents and British forces stationed there have been trading artillery and rocket fire, according to a resident of the area.