SANA, Yemen — Security forces and armed men in civilian clothes clashed with antigovernment demonstrators in the city of Taiz on Tuesday, opening fire on crowds calling for President Ali Abdullah Saleh to step down, witnesses said. It was the third straight day of violence in Taiz, where more than 10 protesters were killed by gunfire on Monday.

Boshra al-Maqtari, one of the protest leaders, said that at least 20 people were wounded by gunfire on Tuesday, but that there had been no deaths.

The clashes began in roughly the same area of the city where the protesters were killed on Monday, the deadliest day so far in six weeks of demonstrations demanding the immediate ouster of Mr. Saleh. As protesters marched down a central street on Tuesday, they encountered the security forces and plainclothes government supporters, and witnesses said military helicopters flew over the demonstration as the violence broke out. Clouds of tear gas rose over the street.

“The thugs and the security forces fired on us with live gunfire,” said Mahmud al-Shaobi, 33, a protester who spoke by telephone and said he was there when the violence began. “Many people were shot.”