At a news conference in Sana, Mr. Saleh claimed that the clashes on Friday were between “citizens and demonstrators” and that “the police were not present and did not open fire.”

President Obama condemned violence in a written statement that called on President Saleh “to adhere to his public pledge to allow demonstrations to take place peacefully.” He added, “Those responsible for today’s violence must be held accountable.”

Friday’s violence came just two days after a team of mediators from Saudi Arabia and other Gulf countries arrived in Sana to try to broker a deal between the government and Yemen’s opposition coalition, which is dominated by an Islamist party.

In his comments on Friday, Mr. Saleh referred to the mediation efforts, saying, “I regret that this event may delay a process that could have saved lives.” But many of the protesters are not affiliated with any established party, and it is not clear that they would have been guided by the deal makers.

Image In recent weeks, many protesters were killed in Aden. Credit... The New York Times

The death toll rose through the afternoon as some of the more than 200 people wounded by gunfire or rocks hurled by government supporters succumbed to their injuries, according to a doctor, Muhammed Rizq, and others at a makeshift hospital near the protest site. The majority of those killed had been shot in the head or neck, doctors said. Many of the wounded were, too, and were expected to die.

Despite the heavy toll, the protesters in Sana kept control of a lengthening portion of Ring Road, which stretches from Sana University to a central highway overpass, as the shooting appeared to halt in the middle of the afternoon.