The security forces used heavy-caliber machine guns and other weapons against the protesters, who had gathered for a peaceful march. At least 15 demonstrators were killed, and dozens of others were injured, witnesses said.
The march was the first in months in which the demonstrators had gathered outside the area of the capital controlled by Maj. Gen. Ali Mohsin al-Ahmar, the second most powerful person in the country, who has sided with the opposition.
The attack on the protesters reflects the recent increase in tensions between the security forces that continue to support President Ali Abdullah Saleh and the tribesmen loyal to his main rivals, the Ahmar family, who are not related to General Ahmar. Mr. Saleh remains in Saudi Arabia recovering from injuries he suffered in a bomb attack in early June.
In the past week, explosions rocked the capital, and tribesmen loyal to the Ahmar family and the president’s forces resumed fighting. There were fears that the latest attack on the protesters would lead to more such violence, and witnesses said the two sides were exchanging gunfire on Sunday evening.
Fighting is also on the rise outside Sana. In Abyan Province in southern Yemen, government forces are battling Islamist militants who are linked to Al Qaeda in the Arabian Peninsula, and north of the capital they are fighting rebel tribesmen.
The president has been out of the country since June, and his prolonged absence has undermined what was already a weak central government.
The violence on Sunday began as the antigovernment demonstrators marched down Agriculture Street, which is on the eastern edge of a long-running sit-in protest that is protected by General Ahmar’s troops. The protesters said they were attacked they moved past the area controlled by the army’s First Armored Division, which is led by General Ahmar.
Men in civilian clothes opened fire from rooftops, the protesters said, and government security forces shot at them from the Ministry of Electricity and, using machine guns, from the backs of pickup trucks. The gunfire lasted about an hour, witnesses said.
A separate group of protesters were met with gunfire and tear gas as soon as they left the area controlled by the First Armored Division, an attack that continued into the evening, witnesses said.
Sakher al-Oldany, a 20-year-old protester, said the marchers moved beyond the area protected by the First Armored Division because they “wanted to escalate” the rebellion against the government. Mr. Oldany, who was hit by tear gas, added, “We weren’t scared, but we were sure this was going to happen.”