Hundreds of tribesmen from rural areas of Yemen have joined the mass protest in the capital, Sanaa, against the regime of President Ali Abdullah Saleh.

Saleh’s security forces launched a vicious attack on thousands of protesters camped near Sanaa University on Saturday. Pro-Saleh thugs attacked the demonstrators with clubs, while police fired live bullets and teargas grenades into the crowd. At least four people were killed in the raid, with hundreds more wounded.

The protesters have held their ground near the university, however, setting up barricades around their camp, which has been joined by more people outraged by the atrocities committed by Saleh’s forces. A spokesperson from the camp’s media committee told the press that an estimated 150,000 people had massed there by Tuesday.

The Ahram Online web site reported that members of some Yemeni tribes have pitched tents on the roads leading towards the anti-government camp in an effort to protect it from further police attacks.

The AFP news agency reported that one tribal leader, Amin al-Akeimi, called on security forces to join “the revolution of the youth.” His Baqil tribal group was “with the youth’s revolution and ready to protect them. We ask the president to leave,” al-Akeimi said.

China’s Xinhua news agency reported that hundreds of thousands of tribespeople from Amran province, just 60 km north of Sanaa, have decided to march on the capital to join the demonstration.

Security forces have responded by attempting to establish road blocks around the university. In an effort to conceal their brutal crackdown on protesters, police detained four Western journalists in the capital on Monday, refusing to let them leave Sanaa International Airport. Dozens of Yemeni and foreign press have been attacked or arrested by police since protests broke out in January.

There are reports of large numbers of protesters being wounded in eastern areas of Yemen. At least 30 people were injured in the province of Marib on Monday as a mass demonstration forced the provincial governor, Naji Ali al-Ziady, to flee the area. The Deutsche Presse-Agentur reported that protesters were fired on with live ammunition as they approached the governor’s office.

Protesters and anti-Saleh tribesmen also seized government buildings in the northeastern province of Jawf.

On Monday more demonstrators were wounded by gunshots and teargas in the major port city of Aden, which has seen some of the fiercest clashes between protesters and the regime.

Tribesmen also protested against the government in the eastern province of Marib on Tuesday, following the bombing of an oil pipeline the night before, believed to have been carried out by members of a local tribe.

The actions in Marib are linked to the killing of a tribal leader in an air strike last year. The United States is working with the Saleh regime to quell tribal insurrections in the country that have been linked to al-Qaeda. The tribes have demanded an investigation into the killing, believing the attack was carried out by an unmanned US drone.

One Yemeni soldier was killed and three wounded in an ambush in the southern province of Abyan on Sunday. The government has blamed al-Qaeda for the ambush, though Saleh faces widespread opposition to his rule across the country.

In a desperate attempt to regain authority, Saleh dismissed one of his cabinet members on Sunday—the first member of his inner circle to be fired since the start of the protests—while several regional governors have been removed from their posts.

Saleh, who has been in power for 32 years, is a key ally of the United States. In the guise of fighting the “war on terror,” Washington has provided military aid to the Saleh regime’s campaign of repression against tribal opposition groups. President Obama has praised Saleh as an ally, and the US administration doubled its aid to Yemen last year.

Though it lacks any major oil reserves of its own, Yemen is viewed as a strategically important country due to its border with Saudi Arabia, the world’s largest oil producer, and location at the Bab-el-Mandeb strait leading into the Red Sea, the route through which millions of barrels of oil are shipped every week.

To defend these interests, Washington has turned a blind eye to Saleh’s murderous campaign against his own people. While the Obama administration uses the pretext of “humanitarian intervention” to drum up support for an air war against Libya, the US government has merely requested that Yemeni authorities show “restraint” and “promptly investigate incidents” of killings.