Two tribesmen were killed in fierce clashes on Thursday in Yemen's capital between troops loyal to embattled President Ali Abdullah Saleh and rival tribesmen and military forces that have defected.

The firefights erupted in north Sanaa between the elite Republican Guard, led by Saleh's son Ahmed, and soldiers of the First Armoured Division, which is led by dissident General Ali Mohsen al-Ahmar and provides protection for anti-Saleh protesters, witnesses said.

Republican Guard forces based in Amran Street were locked in a heavy exchange of fire with dissident troops deployed in Thalathine Street near Change Square where protesters demanding Saleh's ouster have camped for months, witnesses said.

They said heavy shelling believed to be coming from Republican Guard bases north of the capital was targeting a residential neighbourhood near state television, with residents pleading for help and to be spared.

Earlier on Thursday, loyalist troops clashed with Ahmar tribesmen in Al-Hasaba, in renewed fighting with the influential tribe whose leader Sheikh Sadiq al-Ahmar has sided with the protesters.

Two tribesmen were killed and five others wounded, tribal sources said.

The gunbattle erupted a day after other tribesmen fighting the Republican Guard north of Sanaa shot down a fighter jet.

The military held opposition leaders responsible for downing the Sukhoi SU-22 near Arhab, 40 kilometres (26 miles) north of Sanaa, a region that is the northern gateway into the capital.

It also follows a large protest on Wednesday when hundreds of thousands of people demonstrated near Change Square, demanding Saleh's ouster and trial.

Youth groups had said on Wednesday they planned to march from Change Square in north Sanaa to the south of the city where Saleh's residence is located.

"There will be an escalation during the coming two days. The youths will march... to Hedda Street, where the president's residence is," Walid al-Amari, a leading activist from the youth revolution committee, told AFP.

He said protesters want a peaceful march and have asked the leadership of the defected First Armoured Division not to provide any armed protection that could provoke Saleh loyalists.

But the groups appear to have backed off, as demonstrations on Thursday were confined to areas controlled by Ahmar's first division.

Tens of thousands of people marched from Change Square through neighbouring streets before returning to their camp.

"Peaceful. Peaceful. No to civil war," they chanted, an AFP correspondent reported.

Saleh, who is under international pressure to relinquish power and allow new elections, returned to the country on Friday, sparking violence in which scores have been killed.

The 69-year-old president has repeatedly refused to sign a power transfer deal brokered by the Gulf Cooperation Council (GCC) under which he would hand over to Vice President Abdrabuh Mansur Hadi in return for immunity from prosecution.

Hadi warned on Wednesday of civil war in the troubled country, in a meeting with the ambassadors of the permanent member states of the UN Security Council, the European Union and the GCC.

He said the current high level of tension "poses a direct threat to the general situation" in Yemen, the state news agency Saba reported.

"If the situation explodes, the (GCC) initiative would end, as well as all (proposed) peaceful solutions, and Yemen would enter the danger zone of descending into civil war," Hadi said.

In other unrest, one civilian was killed and five others were wounded in overnight bombing in Taez, Yemen's second largest city that is also the scene of continuing massive anti-Saleh protests.