Hundreds of Yemeni tribesmen have joined raging protests at Sanaa University which for weeks have been demanding the departure of President Ali Abdullah Saleh, an AFP correspondent saw Tuesday.
On the roads leading to university square, members of Yemen's tribes pitched tents marked with the names of their home provinces which are concentrated in the north of the trouble-torn country.
Student-led protests demanding the ouster of Saleh, 32 years in power, erupted in the Yemeni capital since late January inspired by the toppling of Arab autocrats in Egypt and Tunisia.
Dozens of anti-regime demonstrators were wounded on Sunday at the Sanaa University square, which has become nerve center of the the capital's popular revolt, as police and regime loyalists unleashed tear gas and bullets in a bid to scatter protesters.
Security forces on Tuesday blocked all of the main arteries leading to the square except the north entrance in an effort to limit the influx of demonstrators.
Tribal chief Sheikh Amin al-Akeimi called on security forces to join "the revolution of the youth" and told AFP the powerful Baqil tribal confederation, which he represents, was "with the youth's revolution and ready to protect them."
"We ask the president to leave," he added.
Some leaders of the Hashid tribe, which is considered Yemen's most influential tribal confederation and includes nine clans, among them the Sanhan, long a bulwark of Saleh's regime, have also joined the ranks of the uprising.
Meanwhile tribal fighters sabotaged an oil pipeline early Tuesday in the eastern Yemeni province of Marib and cut the road between the region's gas fields and the capital, sources said.
The pipeline connecting Marib's oil fields to the Red Sea was an inferno, security officials said.
The fighters also severed the main road between Sanaa and Safer, disrupting tanker traffic and jeopardizing gas supplies to the capital, tribal sources said.
The heavily armed tribes of Marib intended to pressure the government, the sources said, without defining their objective.
Security forces on Monday fired on protesters marching on the governor of Marib's headquarters. Seventeen demonstrators were injured, local officials said.
Yemen's tribes have in the past sabotaged oil pipelines in protest at central government policy.
Governor stabbed, soldiers killed
Anti-government tribesmen in northern Yemen stormed a security building and shot dead four soldiers in a revenge attack after government troops opened fire on opposition protesters calling for the president's ouster, witnesses said.
The attack Monday night in the town of al-Jawf, close to the border with Saudi Arabia, was a significant escalation by the anti-government side in a month of daily street protests in which stone-throwing demonstrators have clashed with security.
Naser Musleh Nasm, a member of the main Islamist opposition, was killed in the armed clash with supporters of the ruling General People's Congress also in al-Jawf province, a security official and tribal leaders told AFP.
On the same day, a provincial governor, Naji al-Zaidi, was stabbed in the neck in the country's east when he attempted to calm the crowd, according to defense ministry website 26sep.net.
However, one protester said the governor was injured during a dispute between security forces and his own bodyguards, four of whom, according to 26sep.net, were also injured.
Also an opposition activist was shot dead in a gunfight with pro-regime loyalists in north Yemen on Tuesday, a security official said.
Yemen, which is already battling secessionist unrest and a Shiite sectarian rebellion, is the poorest country on the Arab peninsula. It is not a member of OPEC but produces some 300,000 barrels of crude oil per day.
Also at least 60 other people were injured in confrontations around the nation between security troops and protesters seeking to topple the country's leader.
A month of protests set in motion by the tumult sweeping the Arab world appears to be spiraling out of control in Yemen, already one of the most impoverished and volatile corners of the Mideast.
Saleh - who has faced down threats from an al-Qaeda offshoot, a secessionist movement in the south and a seven-year armed rebellion in the north - has been unable to stop street protests that are unprecedented in their scope and in the broad cross-section of society taking part.
In a sign of his frustration, Saleh fired the government minister in charge of trying to engage his opponents in dialogue.
Some 40 people have been killed in Yemen's protests that have been marred by violence.
The tribes, which have acted as a counterweight to government power in the past, hold considerable sway in Yemeni society where clan-affiliations remain important.