At least 22 protesters were shot dead and hundreds injured Sunday by troops loyal to Yemeni President Ali Abdullah Saleh at an opposition rally in the capital Sana'a, dpa reported.
The forces opened fired upon the protesters, who were calling for ending the rule of Saleh and bringing to justice those responsible for killing demonstrators in other demonstrations over the past few months.
Snipers positioned on rooftops of buildings shot protesters, according to witnesses.
"The government forces used live bullets against the protesters," said Mona Al-Faqih, a media official for the opposition Al-Taghyeer group.
"Tens of protesters were arrested by police. We have no idea about their whereabouts," she added.
Meanwhile, an unspecified number of people were injured Sunday in renewed clashes between Saleh's Republican Guard forces and loyalists of tribal leader Sheikh Sadiq al-Ahmar in the Sana'a neighbourhood of al-Hasaba, Arab media reported.
In May, confrontations between government forces and armed al-Ahmar tribesmen left some 300 people dead on both sides, with more than 1,000 wounded.
Al-Ahmar has recently vowed not to allow Saleh to keep power.
Saleh is staying in Saudi Arabia recovering from injuries he sustained in an attack on his palace in Sana'a in June.
Since February, millions of Yemenis have been taking to the streets demanding Saleh, who has been in power for 32 years, to step down.