SANAA - Security forces and armed men in civilian clothes opened fire on protesters in the Yemeni city of Taiz, south of the capital, on Tuesday, a day after clashes there killed 15 people, witnesses said.

Anti-government protesters run after police fired tear gas during a demonstration in the southern Yemeni city of Taiz April 4, 2011. Police and armed men in civilian clothes opened fire on anti-government demonstrators in the Yemeni cities of Taiz and Hudaida on Monday, witnesses said, as a drive to oust President Ali Abdullah Saleh gathered pace.  [Photo/Agencies]

Dozens had been wounded, witnesses told Reuters, but there was no word yet from medical sources on casualties.

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Witnesses said hundreds of security troops attacked tens of thousands of protesters, and that men they believed were plainclothes police were wielding bats and daggers. Protesters responded to the attack by hurling rocks at security forces.

Gunfire on Monday in Taiz killed at least 15 protesters and wounded 13.

In the Red Sea port of Hudaida, police and armed men in civilian clothes, whom residents described as government-hired thugs, attacked thousands of protesters who were trying to march on the regional presidential palace.

Six died from gunshot wounds, 30 suffered stab wounds and 270 were hurt from tear gas inhalation.

More than 100 people have now been killed in clashes at protests, and Human Rights Watch on Tuesday called on the United States and other foreign governments to suspend military aid to Yemen in response.