The American efforts here are at a crucial stage, as the United States tries to negotiate a strategic partnership agreement with the Afghans and to pave the way for peace talks. Aware of the damage the Koran burnings can cause, American diplomats and military officials met with Mr. Karzai and spoke to senior Afghan government and religious figures in an attempt to tamp down their anger, said Mark Thornburg, the acting spokesman for the American Embassy.

Among those who met with Mr. Karzai were Ryan C. Crocker, the American ambassador; Gen. John R. Allen, the commander of NATO forces in Afghanistan; and Ashton B. Carter, the deputy defense secretary. They apologized and offered to cooperate fully with the Afghan government in its investigation into what led to the burning of the Korans.

Extremist groups, including the Taliban, were quick to exploit the episode, rallying mobs of mostly young men to confront the Americans and the Afghan security forces. Outside Parliament, a crowd of madrasa students wielding sticks said they were prompted to protest by their teachers. Throughout the morning the highway between Jalalabad and central Kabul was closed by a crowd of at least several hundred people. They set tires on fire and burned checkpoints and a government minibus as they surged toward Camp Phoenix, the NATO military base that faces the road. Many threw rocks at passing S.U.V.’s — symbols of the foreign presence — as well as Afghan police and American military vehicles.