The tribal fighters appeared to consolidate their control of a key district in Sanaa - which includes ministries and the ruling party headquarters - after the fiercest clashes in the three-month uprising against Mr Saleh's authoritarian rule. At least 12 people have been killed over two days of fighting, tribal chiefs and medical officials said.
The decision by Sheikh Sadeq al-Ahmar - head of the powerful Hashid tribe - to unleash his fighters sharply boosts pressure on the embattled president and suggests Yemen could be heading toward a potentially bloody showdown between tribal militias and pro-Saleh troops. Mr Saleh has refused to step down despite three months of nearly nonstop street protests calling for an end to his 32-year rule.
In a clear blow to Mr Saleh's regime, tribal fighters carrying Kalashnikov rifles set up blockades around the Hassaba district, where buildings were pockmarked from gunfire and streets were flooded by water lines blasted by mortars.
The battles broke out after government forces tried to storm Sheikh Ahmar's house. After pushing back pro-Saleh forces, he warned the president to call off his troops or face a harsher battle.
"We are exercising self-restraint," the sheikh said.
Some military commanders have defected to the opposition. But Sheikh Ahmar's move to join the battle could set Yemen on a dangerous path by forcing other clan leaders to pick sides and encouraging military officials to abandon the government.
Mr Saleh refused to sign a US-backed deal offering immunity in return for standing down.