Powerful tribal leaders, including those of the Hashid and Bakeel, pledged to join protests against Saleh at a gathering in Amran province.
"I announce my resignation from the General People's Congress in protest at the attacks on peaceful demonstrators in Sana'a, Taiz and Aden," Hashid tribal chief Sheikh Hussein bin Abdullah Al-Ahmar was quoted as saying, referring to the ruling party.
The Hashid tribe is considered Yemen's most powerful tribal and includes nine clans, among them Sanhan, long a bulwark of Saleh's regime.
The announcement was warmly received by a large crowd of tribesmen, including members of Yemen's second largest tribe, Bakeel, who gathered for the meeting, according to the tribal source.
The two tribes announced they would support the popular uprising against Saleh, who refused to step down after three decades in power. Protesters main chant has been "the people want the fall of the regime!"
On the other hand, Sheikhs of Hashed and Bakel Tribes denied in a statement posted on the Internet Al-Ahmar allegations of joining protests against Saleh's regime. ''We announced our stance in a meeting held earlier this month with President Saleh. Our stance is with security and stability of the country,'' the statement said.