Government snipers in Yemen shot and killed at least 40 people on Friday.
The snipers reportedly shot into a crowd of demonstrators who were calling on President Ali Abdullah Saleh to step down. Hundreds of people were injured.
Saleh declared a state of emergency shortly after the violence. He denied that his security forces were involved in the shooting.
The latest violence is being called the harshest government response since the uprising began in Yemen a month ago.
President Barack Obama condemned the latest attack.
In a written statement the president called on Saleh to, "Adhere to his public pledge to allow demonstrations to take place peacefully."
"Those responsible for today's violence must be held accountable," Obama added.