"There will be more ... bigger protests," said Shahid Shamsi, spokesman for an alliance of Islamic groups.
"Pakistani civilians, including children, were killed," he said. "Principles cannot be broken in the name of (fighting) terrorism."
Protesters believe Friday's attack was the work of U.S. forces in Afghanistan. Washington has 20,000 soldiers in that country searching for Taliban and al-Qaida fighters, but Pakistan says it has not given the Americans permission to pursue their enemies across the border.