At least 20 people were killed after forces loyal to Yemeni President Ali Abdullah Saleh opened fire on protesters in the southern city of Taiz. At least 150 people were reported to have been injured in the clashes. Security forces also set the protest camps on fire.
Protesters claimed that troops backed by tanks had stormed a field hospital, detaining some of those who had sought treatment there.
"This was a massacre. The situation is miserable. They have dragged the wounded off to detention centers from the streets," activist Bushra al-Maqtari told news agency AFP.
The opposition coalition condemned what it called the "crimes against humanity" committed by Saleh's "remaining military and security forces and armed militias."
It warned the president he would be "held personally responsible for his continued crimes against the people."
On Sunday night, police fired live ammunition, tear gas and water cannon to disperse some 3,000 demonstrators outside a municipal building who were demanding the release of a fellow protester. They have been staging the sit-in for four months, making it the longest-running protest against Saleh's rule.
More than 200 demonstrators have been killed since protests first erupted in Yemen in the spring. Scores more people have died in armed clashes between loyalist troops and dissident tribesmen.
Author: Joanna Impey (AFP, dpa, Reuters)
Editor: Nicole Goebel