At least 40 people have been killed in Yemen amid fierce fighting between troops loyal to president Ali Abdullah Saleh and his military and civillian opponents in the capital, Sana'a.

Around the city, government forces fought on three fronts against a rebel army division, tribal fighters and pro-democracy demonstrators. The violence raged a day after Saleh's sudden return to the country on Friday, which has prompted fears of all-out civil war.

At least 11 soldiers were killed and 112 were wounded when government forces shelled the headquarters of the renegade 1st Armored Division, according to pro-opposition officer Abdel-Ghani al-Shimiri.

There have been street battles and exchanges of shelling all week between Republican Guards led by Saleh's son and the rebel division led by Major General Ali Mohsen al-Ahmar.

On Saturday Al-Ahmar described Saleh as a "sick, vengeful soul" set on driving the country into civil war, and called on the international community to rein him in.

Earlier, 29 people were killed as Republican Guards led by Saleh's son, Ahmed, attempted to clear the pro-democracy camp on Change Square, which has been the focal point of the opposition protests to oust the president for the past seven months.

Another 54 people were wounded in the attack on the tented shantytown, said Mohammed al-Qabatis, a medic at the field hospital set up in the square. "More bodies and injured are pouring into the hospital," the doctor added.

Qabatis said continuing heavy fighting meant ambulances were forced to leave many bodies in the streets. "Many of the injured get here by motorcycles," he said.

Republican guards used anti-aircraft guns, automatic weapons and rocket-propelled grenades against the opposition camp and intensified the shelling of the streets surrounding the square.

Protesters distributed plastic helmets in a desperate effort to protect themselves from the bullets of pro-government snipers prowling the rooftops nearby. Witnesses said several houses were destroyed by mortar shells.

On the third front in the capital, Saleh's troops clashed with pro-opposition tribal fighters around the district of Hassaba. At least 18 tribal fighters have been killed in clashes there over the past two days, according to a statement from tribal elders today.

Hassaba is home to Yemen's most powerful tribal confederation, the Hashid, led by Sheikh Sadeq al-Ahmar, another opponent of Saleh.

When the president was airlifted to Saudi Arabia for treatment after an explosion at his compound in June, al-Ahmar swore "by God" that he would never let Saleh rule again.

The last time hostilities between the Saleh and Ahmar families turned violent in May, a week's worth of mortar battles erupted, flattening an entire neighbourhood in east Sana'a and killing hundreds.

Yemen's interior minister, General Mouthar al-Masri, said eight government troops were killed and dozens of others were wounded. It was not clear if he was referring to violence in Sana'a or elsewhere. More than 100 protesters have been shot dead in the past week.

The abrupt reappearance of Saleh, who has spent the past three months recovering in Saudi Arabia from an assassination attempt, has created fresh confusion around tentative hopes for a way out of Yemen's political crisis. The president's return came as a shock, with western diplomats in Sana'a and even members of his ruling party seemingly caught unawares.