Sheik Hamoud al-Makhlafi is prepared for war. Inside his palatial house, overlooking this tense city, more than 20 of his tribesmen stand guard with Kalashnikov rifles and rocket-propelled grenades; hundreds of other armed fighters are hidden nearby in neighborhoods wrested from government control.

“We are here to protect the youth of the revolution,” declared Makhlafi, 46, slim with short, gray-speckled hair and a razor-thin mustache.

But the intervention of tribal militias in what had been a nonviolent revolution has added a combustible new dimension to the uprising in Yemen. Portions of Taiz, Yemen’s second-largest city, have turned into a war zone, and while the tribesmen say they are protecting the activists, the change appears likely to bring more upheaval to this fractured Middle Eastern nation.

Last weekend, violent clashes erupted again between the tribesmen and government forces, which included shelling of parts of the city, killing several people.

“Taiz is more of a time bomb now. It could explode at any minute,” said Ali Mohammed Almujahed, a senior ruling party official here. “Both sides are filled up with anger and hatred.”

Fueled by decades of neglect and resentment of President Ali Abdullah Saleh’s regime, protesters in this south-central city, considered the intellectual soul of Yemen, rose up in February. Inspired by the uprisings in Tunisia and Egypt, they erected scores of tents in a section of Taiz they renamed Freedom Square, emulating Cairo’s Tahrir — or Liberation — Square. Thousands camped out around the clock, staging rallies demanding the ouster of Saleh.

Since April, the government’s security forces have tried to suppress the rebellion, shooting dead dozens of protesters and preventing them from marching toward the city’s presidential palace and other government institutions. The tactics in recent weeks have emulated those used by autocrats in Syria and Bahrain, who have cracked down violently on protesters in an effort to intimidate the opposition.

May 29: A touchstone

For weeks, Makhlafi and other anti-government tribal leaders expressed support for the protesters but watched from the sidelines. Some were aligned with Yemen’s political opposition; others held long-standing grievances against the regime and sensed an opportunity to exert their power.

Then, on May 29, the security forces attacked Freedom Square. There are conflicting versions of what unfolded, but it remains a touchstone for all sides.

Activists say the soldiers opened fire on demonstrators and set fire to tents. As many as 140 were killed, the activists allege. They now refer to the day as “the Holocaust.” In a rare interview, Taiz’s head of security, Col. Abdullah Abdu Kayran, denied the allegations. He acknowledged that security forces entered the square but said they did not commit atrocities. He said eight people died that day.

Within hours of the attack, Makhlafi and other tribal leaders rose up and engaged the security forces in fierce clashes. The tribesmen hailed from villages on the outskirts of the city. Armed with mortars, rocket-propelled grenades and Kalashnikovs, they knew the city’s streets and alleys well and had the loyalty of much of the population. That gave them a significant advantage over the security forces that were largely brought in from other parts of the country.

Buildings were destroyed; bodies lay on the streets. The tribesmen took control over large portions of the city, including 14 government buildings. The fighting intensified after Saleh was nearly assassinated in a June 3 attack on his presidential compound in the capital, Sanaa, forcing him to receive treatment in neighboring Saudi Arabia.

“Those who perpetrated the Holocaust should be confronted with the same weapon,” said Sultan al-Samei, an influential tribal leader and opposition socialist lawmaker who dispatched his forces alongside Makhlafi’s. “It’s our natural right to defend ourselves.”

Mistrust kindled

The muscle-flexing by a powerful anti-government tribe has kindled some mistrust among protesters and government loyalists alike.

Government officials describe Makhlafi as a pawn of Yemen’s traditional political opposition, in an effort to violently overthrow the government under the guise of protecting the demonstrators. Some youth leaders say they worry that the tribes could take over their populist rebellion and harness it to gain political influence.

But the youth leaders also say that without the protection of the tribes, their uprising would be stamped out violently by the regime.

“We are quite aware of the possibility that they could hijack our revolution, and we are keeping careful watch of this,” said Manal Abdulrahman, 25, an unemployed university graduate and activist. “Right now, we need them to get rid of President Saleh, his sons and nephews.”

A law graduate, Makhlafi once served as a political security officer in Saleh’s regime. He ran for local office in the 2006 elections but did not win. When the uprising began in February, he defected.

Among government officials, Makhlafi’s opponents describe the tribal leader as ambitious and ruthless. “His personal profile is full of crimes, killings and troublemaking,” said Mohammed Mansour al-Shawafi, the deputy governor of Taiz province. “He has no idea of democracy.”

But Makhlafi says he has no political aspirations. “I am not affiliated with any political party,” he said. “I am not interested in power.”

Today, the city is gripped by a tense stalemate. A cease-fire negotiated by local leaders calls for the tribal militias and the security forces to withdraw from the city. But neither shows any signs of observing the pact.

Security forces still maintain a heavy presence in the city, manning checkpoints. Both sides trade accusations. Clashes still unfold and have taken on an ominous new dimension. “They are now targeting my tribe,” Makhlafi said, adding that six of his tribesmen have been killed and eight injured in recent weeks.

Under the terms of the cease-fire, Makhlafi’s men have recently handed back eight government buildings but continue to hold on to the electricity department and judiciary complex. Makhlafi says they will not leave until the security forces withdraw to their barracks outside Taiz; Kayran, the local security chief, says that approach is out of the question.

Freedom Square today is virtually empty. It was once covered by a sea of tents, but only a few remain. The protests are still unfolding, but with smaller numbers and less intensity. Activists say the security forces continue to fire sporadically into the square, scaring protesters away. Massive fuel shortages are also preventing people from coming to the square. Government spies keep watch.

Youth leaders are determined to resurrect the momentum. Security and other committees are being reorganized. Facebook campaigns are underway to bring back protesters. “Taiz is still betting on the fall of the regime,” said Bushra al-Maktari, a protest leader. “The revolution is still here.”

Still, the soul of the uprising appears to have been permanently altered. Makhlafi said that at every demonstration now, his tribesmen are hidden in the crowds, armed and ready for any possible attack by security forces.