Dozens of chiefs from Yemen's two largest and most powerful tribes called on the president to immediately step down and strip his son of control over security forces, as rival crowds of protesters took to the streets Friday.
The abandonment of President Ali Abdullah Saleh by the two tribes, including one that is linked to his own, is part of a larger crumbling of support for the autocratic leader after weeks of protests against his rule. Several army commanders, university professors and religious leaders have also defected to the opposition calling for his ouster in near daily protests since mid-February.
Saleh has, nonetheless, held on, refusing to end his 32 years in power and waging a crackdown that has killed more than 120 people, according to Yemeni rights groups.
The United States is among world powers watching the unrest with concern, in particular, because of the uncertainty over who would replace Saleh and whether a new Yemeni leader would be willing or able to continue cooperating with Washington in battling al-Qaida's most active offshoot, which operates out of Yemen.
President Saleh has warned that al-Qaida in the Arabian Peninsula, which the U.S. estimates numbers about 300 fighters, would take control of the country if he stepped down.
The tribal chiefs who have joined Saleh's political opponents sought to reassure the world Friday that leaders from the opposition movement would "combat terrorism and dry the sources of its flow."
The 100 chiefs from the Hashid and Bakeel tribes released the statement after two days of meetings with religious leaders, university professors and rights activists to discuss the crisis. Saleh's Sanhan clan is affiliated with the larger Hashid tribe.
Leaders of both tribes had already announced their support for the opposition, part of a wave of defections triggered by the single bloodiest day of the government's crackdown, when snipers killed more than 40 protesters in the capital, Sanaa, on March 18.
In their statement Friday, they called on Saleh to step down "immediately and sack his sons and relatives from their influential posts at the security apparatuses and the army."
Hundreds of thousands of people held protests against Saleh's government after Friday's Muslim prayer services outside Sanaa University. There were also massive protests in the southern city of Taiz, the port of Aden and in eastern Hadramawt province.
Saleh rallied tens of thousands of his own supporters near his office, telling the gathering that their numbers demonstrated his legitimacy and signaled a "rejection of chaos."
Yemen is the poorest country in the Arab world, and protesters are calling for steps to improve livelihoods and open up the country's restricted political life. The protests are the biggest direct challenge to Saleh's three-decades in power. His weak government has little control beyond the capital and has struggled to confront an armed rebellion in the north and a secessionist movement in the south.
Neighboring Saudi Arabia, whose leaders have also been targeted by al-Qaida in the Arabian Peninsula, has been among six Gulf nations trying to broker a solution to the crisis.
It has proposed that Saleh hand power to his deputy in exchange for immunity from prosecution for any crimes committed under his leadership. It specified no timeframe for his departure.
Both sides rejected the proposal. The opposition wants him to go immediately and to face a legal reckoning, and Saleh refused to leave before the end of his term in 2013.
The Gulf nations urged the two sides to meet Saturday in the Saudi capital, Riyadh.
Opposition spokesman Mohammed al-Sabri said Friday that "the president should first step down and hand over his authorities to his deputy and then the opposition will go any place in the world" for talks.
Later Friday, thousands of women, most of them cloaked head-to-toe in black robes, marched in Sanaa, Aden and several other cities to denounce comments Saleh made against the participation of women in protests alongside men.
Referring to the protests in the capital, Saleh said during his Friday speech that the "mingling of women and men at Change Square is forbidden and against Sharia (Islamic law)."