Armed clashes broke out Monday between Yemeni policemen and tribesmen loyal to opposition chieftain Sadiq al-Ahmar in Sanaa, leaving several people wounded, witnesses and a security official said.
Machine guns were fired and grenades hurled during the clashes, which happened near Ahmar's home in the Yemeni capital, according to witnesses.
Several people were wounded in the fighting, the security official said on condition of anonymity, without specifying how many or from which side.
Ahmar, who leads the Hashid tribal federation, the largest in deeply tribal Yemen and a former crucial source of embattled President Ali Abdullah Saleh's power, in March pledged his support for the opposition.
"I announce in the name of all the members of my tribe that I am joining the revolution," Ahmar said, calling for Saleh "to exempt Yemen from the bloodshed and make a quiet exit."
Monday's fighting came after Saleh on Sunday refused to sign a Gulf-brokered transition plan that would have seen him cede power to his vice president within 30 days, in exchange for immunity from prosecution for himself and his aides.
The strongman, who has been in power since 1978, instead demanded the opposition ink the accord in his palace.
Since late January, security forces have mounted a bloody crackdown on anti-regime protests, leaving at least 180 people dead, according to a toll compiled from reports by activists and medics.