Tuesday's attack in the town of Baghlan, some 153 kilometers (95 miles) north of the capital, Kabul, was the deadliest since the U.S. toppled Taliban forces in 2001.
Karzai said the bombing was carried out by "the enemies of peace and security," an Afghan government euphemism for the Taliban and al-Qaeda linked militants. The Taliban has denied any involvement, and no other groups have claimed responsibility.
The bomb exploded as schoolchildren, tribal elders and government officials lined the streets of the town to greet lawmakers who had come to visit the town's cement and sugar factories.
Eighty-one people were also wounded in the attack, at least 42 of them children. It was not immediately clear how many children were among the dead.
The White House called the attack "a despicable act of cowardice."
"It reminds us who the enemy is - extremists with evil in their hearts who target innocent Muslim men, women and children," press secretary Dana Perino said.
Russia said that it condemned the "barbaric and inhumane act," and conveyed its condolences to the Afghan government, as well as to the friends and family of those who died in the blast.
