SAN'A, Yemen - Fighting spilled across Yemen's capital Thursday, and frightened residents fled or cowered in basements as a powerful militia alliance warned President Ali Abdullah Saleh to either step down or face civil war. At least 28 people were killed as the four-day death toll neared 110.

The bold ultimatum reflected the growing confidence among the opposition forces - led by Yemen's largest tribe - that they could be gaining the upper hand against Saleh's regime with the uprising shifting from near-daily street protests to fast-moving urban combat.

But Saleh has shown he will not go easily. He has managed to ride out swelling antigovernment demonstrations for more than three months, defections of military commanders, and pressure from Arab neighbors and Western powers to leave office.

The difference this time could be the vast influence of Yemen's main tribes, which command well-armed militias and can tap into traditional clan-based loyalties to try to bring down Saleh's 32-year rule.

In the latest battles, government forces pounded tribal fighters with artillery and mortar rounds even as fighting fanned out around the capital, San'a, to areas that included the airport zone. Hundreds of Yemenis raced out of San'a or took refuge in basements to escape mortar strikes.

Fighting also flared in other hot spots around the country, which has the world's most active al-Qaeda-inspired group.

But there was no indication that Islamic extremists were joining the offensive.

"The tribes are all moving against the president and have a golden opportunity to push Saleh to step down," said Faris Sadqaf, a Yemeni political analyst.

Under Yemen's ancient codes, tribal leaders can declare that members follow their orders above all others. This potentially gives tribal chiefs the power to order government soldiers from their clans to stand down.

There were no apparent signals of mass defections from Saleh's military. But some key commanders have jumped to the opposition, and the leader of Yemen's largest tribe, Sheikh Sadeq al-Ahmar, warned that Saleh had the option to step down or be held accountable for "dragging the country to a civil war."

The battles broke out Monday after an attempt by government forces to storm Ahmar's compound in the heart of San'a.

By Thursday, the clashes had widened to include areas around the San'a airport.

At least 109 people have been killed, including at least 28 Thursday.