At least 18 people were killed and 35 others wounded Friday in fierce clashes that rocked Yemen's capital Sanaa hours after President Ali Abdullah Saleh returned from Riyadh and called for a ceasefire between his troops and foes, opposition media said.

"At least 18 people were killed and 35 others wounded in the government's heavy shelling (that) targeted strongholds and the residential compound of dissident powerful tribal leader Sadiq al- Ahmar in the north Sanaa district of Hassaba," said Al-Sahwa.net, the news website of Yemen's main opposition party Al-Islah.

A Xinhua reporter at the scene said heavy fire from machine guns and mortars was exchanged in Hassaba Friday evening between the government troops and the armed tribesmen of al-Ahmar who were backed by units of the defected army.

The defected army were commanded by General Ali Mohsen al-Ahmar, who is also a half brother of President Saleh. He deserted Saleh in March and supported the protests demanding an immediate end to the 33-year rule of Saleh.

The Defense Ministry on its website accused al-Ahmar and the defected forces of shelling the government military camps and headquarters of the Interior Ministry in Hassaba, blaming them for "breaching the ceasefire called by President Saleh."

Saleh, who has confronted eight months of protests against him, made his call for a truce and dialogue hours after his return from the Saudi capital Riyadh, where he had spent over three months to treat injuries sustained in an "assassination" bomb attack on his palace in Sanaa.

The Yemeni government blamed the attack on Sadiq al-Ahmar and Ali Mohsen al-Ahmar.

Al-Sahwa.net meanwhile quoted Sadiq al-Ahmar as saying "I'm still committed to the truce, but if (President) Ali Saleh continues his aggressions, then my patience will fade."

The street battles in Sanaa flared up Sunday, and a truce deal mediated Tuesday by Yemeni Vice President Abd-Rabbu Mansour Hadi was broken, increasing the death toll in the capital to 111 and leaving at least 1,000 people injured, according to officials affiliated with the rivals.

Saleh's surprise return shocked his opponents, but largely delighted his supporters. Tens of thousands of Saleh supporters gathered after Friday's midday prayer in a square near the presidential place in eastern Sanaa.

Reported statements by officials of the ruling party have reaffirmed that "Saleh will not quit until his presidential term expires in 2013."

Tens of thousands of anti-government protesters also staged a rally Friday in a square in western Sanaa under heavy protection provided by the defected army.

"Saleh came back to be prosecuted for his massacres," the protesters shouted.

Opposition spokesman Mohamed Qahtan told Xinhua that "Saleh's return is none of our business ... The protest movement will go ahead to oust him."

The defected army has not made any comment on Saleh's return.