But their uprising has been hijacked by Yemen's two traditional powers -- the tribes and the military -- all but ensuring that even if a new regime emerges from the chaos, it will not look or act much differently from the old one.

"Today, our revolution is at a crossroads," said protester Mansour Hamed. "It can either triumph through peaceful means or the whole nation will slide into civil war, in which case the military and the tribes will have stolen our revolution."

Breakaway military units and tribal fighters have been battling for more than a week in the capital with troops loyal to President Ali Abdullah Saleh, in fighting that has escalated since the president returned last week from Saudi Arabia, where he had been undergoing treatment for nearly four months for wounds suffered in an assassination attempt. As a result, turmoil has deepened in this divided nation, where the United States wants to preserve a focus on fighting al-Qaida militants.

The ruinous urban warfare has caught protesters in the middle -- and as a result they make up the bulk of more than 150 people killed in the fighting.

Moreover, it is threatening to turn the protest movement into a sideshow. The tribal chiefs and army commanders who switched sides and joined the opposition have already taken over an uprising that initially was engineered mostly by pro-reform youth groups with a vision for a democratic and free Yemen.

Inspired by popular uprisings in Tunisia and Egypt, Yemen's peaceful protests brought together a diverse mix of people in massive demonstrations -- sometimes drawing millions -- across this impoverished Arab nation. They shattered Yemen's entrenched gender, tribal and age barriers in a rare show of unity in a patriarchal and deeply religious society where tribal loyalties prevail and regional rivalries are common.

"The revolution has given birth to a whole new generation that has become aware of its potential," said Magid al-Madhaji, a protest leader and rights activist. "Their resistance to Yemen's traditional forces will continue."

The heavy hand of the military and tribes has already weakened the protest movement. The mostly liberal, left-leaning Yemenis of the south, for example, reduced their profile in an uprising they see as now led by Islamists and army commanders who have persecuted them for years.

"The traditional forces are delaying our victory," said Lina al-Hosny, an opposition activist from the southern port city of Aden. "If this continues, the revolution will fail."

Over the past week, Saleh's loyalists have shelled crowds of demonstrators, and bombardment exchanges have flown back and forth over Sanaa, the capital, between the elite Republican Guard troops led by Saleh's son Ahmed and the troops of renegade army general Ali Mohsen al-Ahmar.