Yemeni tribes backing a nine-month protest movement against President Ali Abdullah Saleh clashed with his loyalists on Monday in Sanaa, witnesses said, after weekend gun fights left more than 20 dead.

The firefight in north Sanaa began after midnight and continued into the morning in al-Hassaba neighborhood, the base of tribesmen led by Sheikh Sadeq al-Ahmar, a fierce foe of Saleh, residents said.

Fierce clashes erupted in the Yemeni capital between troops loyal to President Ali Abdullah Saleh and rival forces on Saturday, a day after the U.N. urged the embattled leader to hand over power.

Witnesses and AFP correspondents in Sanaa said explosions were heard throughout Sanaa from early hours and massive plumes of smoke and fire seen rising from several neighborhoods where opposition forces are stationed.

The Yemeni opposition called on the United Nations on Thursday to force President Ali Abdullah Saleh to step down unconditionally, rejecting his request for international guarantees.

The opposition Common Forum dismissed as a sham Saleh's announcement on Wednesday that he was ready to sign a Gulf-brokered deal for him to quit office in return for a promise of immunity from prosecution if it was backed up by European and U.S. guarantees.

Yemeni troops shot dead four protesters Sunday as they attempted to block a new bid by activists to bring 10 months of protests against veteran President Ali Abdullah Saleh to a head.

The defiant march by tens of thousands of pro-democracy demonstrators came despite the deaths of at least 12 people in a similar protest in the capital on Saturday.

Yemeni pro-democracy activists called for a new mass demonstration Sunday aimed at bringing 10 months of protests to a head, a day after troops shot dead at least 12 people marching in the capital.

"We will continue with our protests... even if thousands of our youth are killed. This is the only way to ensure the fall of the regime," Walid al-Ammari, a spokesman for the protesters, told AFP.

Demonstrators on Friday called for the United Nations to intervene to put Yemeni President Ali Abdullah Saleh on trial in the wake of a crackdown on anti-regime protests that has cost hundreds of lives.

"We want the world to pass a resolution which defends the blood of the revolutionaries," protesters chanted at a huge gathering near "Change Square" in central Sanaa that has become the epicenter of a campaign to oust Saleh.

Shells fired into a popular shopping district of Yemen's capital killed one civilian and wounded two others on Tuesday, witnesses and medical officials said.

The two shells slammed into Hayel Road late in the morning as the area was packed with shoppers and pedestrians, killing one man, the witnesses said.

Fierce clashes erupted in the Yemeni capital on Thursday between troops loyal to embattled President Ali Abdullah Saleh and forces led by defected General Ali Mohsen al-Ahmar, witnesses said.

The firefights broke out in the north of the city between forces from the elite Republican Guard, led by Saleh's son Ahmed, and soldiers from Ahmar's First Armored Division, which provides protection for anti-Saleh protesters.

At least 40 people, among them dissident soldiers, were killed in violence that rocked Yemen's capital on Saturday, an activist from the protest organizing committee told AFP.

"More than 40 people were killed on Saturday" in battles that hit several neighborhoods across Sanaa, including Change Square, epicenter of anti-regime protests since February, the activist said.

Troops loyal to Yemeni President Ali Abdullah Saleh killed at least 17 people in an assault early on Saturday in the capital Sanaa, witnesses and a medic told AFP.

"Seventeen people were killed and 55 others were wounded" in the attack, said Mohammed al-Qabati, a medic at the field hospital in Sanaa's Change Square.