At least 37 people have been killed in overnight fighting in the Yemeni capital Sanaa, doctors and officials say.
Violence escalated after a ceasefire broke down between security forces and fighters loyal to Sheikh Sadiq al-Ahmar, head of a tribal confederation.
Analysts say the conflict threatens to drag Yemen into civil war.
Yemen is facing increasing unrest in several areas after President Ali Abdullah Saleh refused to step down.
Witnesses reported heavy fighting in the capital overnight as both sides blamed each other for breaking the ceasefire.
The defence ministry accused tribesmen of seizing the headquarters of the ruling General People’s Congress and other offices in Sanaa.
However, sources close to Sheikh Ahmar said government forces had attacked his compound in the north of the city.
Medical officials in Sanaa said casualties included fighters from both sides.
On Tuesday, another 12 protesters were shot dead in the southern city of Taiz following a move by security forces to remove the protest camp from the city centre.
There were also further clashes in the town of Zinjibar where government forces have been battling fighters described as al-Qaeda militants.
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