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SANAA, Yemen (AP) — Security forces battled in the streets of the Yemeni capital with fighters from the country’s most powerful tribe, which has joined the opposition, in an eruption of violence Monday after President Ali Abdullah Saleh refused to sign an agreement calling on him to step down.

The fighting was the fiercest yet between the pro- and anti-Saleh camps, raising fears that the collapse of efforts to negotiate a peaceful resolution to Yemen’s 3-month-old crisis could throw the country into a violent confrontation. Hundreds of thousands of Yemenis have been holding protests since February demanding Saleh’s removal, and they have been hit by a bloody crackdown — but before Monday there had been only minor armed clashes between the two sides.

The violence erupted outside the Sanaa home of Sheik Sadeq al-Ahmar, leader of Yemen’s largest tribe, the Hashid. Saleh himself belongs the tribe, but al-Ahmar announced in March that the Hashid were joining the popular uprising against the president.

The spark of the fighting was unclear. Some witnesses said security forces had been setting up roadblocks between al-Ahmar’s walled residential compound and the nearby Interior Ministry, and that tribesmen saw it as a provocation.

Tribesmen and security forces battled in the streets surrounding the ministry, trading fire with automatic weapons and rocket-propelled grenades. Smoke was seen rising from the ministry building. Tribal fighters took over buildings belonging the nearby Industry Ministry as well, the Interior Ministry said in a statement.

Hundreds of Hashid fighters were moving in from outside Sanaa to al-Ahmar’s house to protect it, a tribal offical said, speaking on condition of anonymity because of the sensitivity of the situation.