At least three people were killed here in the capital, and a 14-year-old boy was killed in a protest in the south, witnesses said.
The American ambassador here, Gerald M. Feierstein, warned that the situation had become “dangerous” and urged both sides to pursue negotiations.
The confrontations came a day after the largest protest yet in the three-week uprising, when an estimated 100,000 people staged a sit-in in Sana demanding the ouster of Mr. Saleh, who has ruled for more than three decades.
The numbers had dwindled to about 10,000 by early Saturday morning, when the police moved in with water cannons to try to demolish tents set up by protesters outside Sana University. Then the shooting started.
“We finished our morning prayers, and then they rushed at us with a water cannon and then live bullets and tear gas,” said Sadeq al-Haijazy, an unemployed protester, his head bandaged after he was hit by a thrown rock. “But we are staying here until we die.”
Plainclothes pro-government forces with Kalashnikovs fired from the rooftops and upper floors of nearby apartment buildings. At least some of the shooting appeared to be with rubber bullets.
Some protesters retaliated by throwing rocks toward the security forces, which included about 200 soldiers and plainclothes forces, breaking pieces of stone from a barrier that soldiers had previously used to block the protests.
“They wanted to push us back off this street,” said Abdel Kader al-Mamari, an engineer standing in a puddle left over from the water cannon barrage. “They don’t understand that what we want is to change the regime.”