The fighting that pitted Yemen's most powerful tribe against security forces still loyal to the embattled leader was the bloodiest since protests against Mr Saleh began in January. At least 28 people were killed yesterday, bringing the death toll for this week to nearly 100.
It also forced hundreds of people to flee from the city, strapping possessions to the roofs of their cars. Explosions could be heard in Sanaa near the protest site where thousands of anti-Saleh protesters are camped and huddled around televisions watching for developments.
The latest round of fighting began when the home of the country's most prominent tribal leader, Sadeq al-Ahmar, came under fire from security forces. It prompted fighters from his tribal confederation to attack ministries and try to take them over along with other key installations.
Known as a "black shame" in Arabic, tribesmen said that the shelling of Mr Ahmar's home during discussions about a ceasefire was a declaration of war. The mansion was littered with broken glass yesterday, and its corridors were marked with blood. It had also become a makeshift clinic for the wounded.
Mr Ahmar called on fellow tribesmen to come to the aid of those in Sanaa to help bring down Mr Saleh. "Ali Abdullah Saleh is a liar, liar, liar," he said. "We are firm. He will leave this country barefoot."
Mr Saleh has vowed to remain steadfast after a series of inconsistent statements about his position since the beginning of the crisis, which has seen parts of the army defect to the youth-led rebellion. The regime has struck back and issued arrest warrants for all 10 Ahmar brothers.
The most recent bout of fighting erupted a day after Mr Saleh pulled out for the third time from a deal mediated by Gulf Arab neighbours for him to quit and make way for a unity government. US President Barack Obama has called for Mr Saleh to sign the deal but analysts said Washington has little leverage in Yemen even though it has sent about $300m in aid to help prop up Mr Saleh's government.
Pressure has been mounting on the regime since February, when protesters inspired by democratic revolutions in Tunisia and Egypt began camping in squares and marching in their hundreds of thousands to call for Mr Saleh to go. His attempts to stop the protests by force have so far claimed the lives of 260 people.
The United States and Saudi Arabia, both targets of foiled attacks by a wing of al-Qa'ida based in Yemen, have tried to defuse the crisis and stem any spread of anarchy that could give the global militant network more room to operate. There are worries that Yemen, already teetering on the brink of financial ruin, could become a failed state that would undermine regional security and pose a serious risk to its neighbour Saudi Arabia, the world's biggest oil exporter. As the fighting increased, the Foreign Office and the US both asked their citizens to leave the country. The US also ordered non-essential diplomatic staff out of the country.
At a square near the university where anti-Saleh protesters have remained camped for weeks, the normally jovial atmosphere was much more sombre as the call the prayer rang out amidst the echoes of gunfire and shelling.
Witnesses from the Al-Hasaba district of Sanaa, where the fighting has been concentrated in the past four days, said that government troops have beaten back the tribal fighters outside the city. "Yes, they've been beaten back," said one resident, "but I fear they will return in greater numbers".
However, the situation remains tense as sporadic gunfire and shelling is still heard. There were long queues at bakeries, banks and petrol stations as residents tried to stock up on cash and food before fleeing to safer areas. Several electronics and clothes shops opened but few buyers were around except those shopping for food.
"We can't leave the house and we can't buy food or water. We're completely under siege," said Ishtar, one resident holed up in her family home in Al-Hasaba.
Less a country, more a collection of tribes
Modern Yemen was born of the union of its North and South in 1990, but allegiances in the desperately poor country are largely governed by tribe, with hundreds of different ones at the south-western tip of the Arabian peninsula.
The most powerful is the Hashid tribal confederation, and its prominent sheikhs have slowly come out in support of Yemen's protest movement. Hashid, while a fractured union of smaller sheikhs and villages, is by and large led by 10 brothers. They are sons of Yemen's most beloved and powerful sheikh, Abdullah al-Ahmar, who pledged his support to President Ali Abdullah Saleh following Northern Yemen's revolution against a tyrannical imam in 1967.
Abdullah died in December 2007, passing his title as head of Hashid to his eldest son, Sadeq. Known as the kingmakers, Abdullah's sons may be willing to help in yet another attempt to overthrow a dictator.
Hamid al-Ahmar, Abdullah's most outspoken and charismatic son, is a millionaire businessman, a member of Yemen's political opposition Islah party, and a long-time Saleh detractor. He was the first of the brothers to publicly side with Yemen's anti-government protest movement in February.
Sadeq followed suit in March after more than 50 protesters were killed as plainclothes snipers opened fire. It was outside Sadeq's home that the fierce fighting broke out on Monday, setting off a chain of events that has brought the country to the brink of all-out armed conflict.
The Hashid are heavily armed and experienced fighters. Since the rule of Imam Yahya in the early 20th century, neither imam nor president has been able to defeat the tribes militarily. Mr Saleh has maintained a fragile hold over the tribes through stipends and government appointments. But at this point, he has lost all tribal loyalty.
Ironically, Mr Saleh's own Sanhan tribe is a member of the Hashid confederation. But Sadeq al-Ahmar has made it clear that Mr Saleh is no longer considered one of their brothers.
