Tribesmen fighting Yemeni troops loyal to President Ali Abdullah Saleh shot down on Wednesday an army fighter jet, as a sea of protesters demanded the under-fire leader's ouster and trial.

A Sukhoi SU-22 ‘fell during a regular mission’ and opposition leaders were ‘responsible for the incident,’ said a military spokesman quoted by Saba state news agency.

Meanwhile, hundreds of thousands of people demonstrated near Sanaa's Change Square, the focal point of anti-regime protests in the violence-wracked Arabian Peninsula country, a correspondent reported.

"We shall not rest until the butcher is executed," the demonstrators chanted as they marched in a neighbourhood of the capital controlled by dissident General Ali Muhsen al-Ahmar's First Armoured Division.

Security forces loyal to Saleh blocked the road leading to government offices beyond the Qiyadah roundabout, forcing the protest to stay within the area controlled by the defected division.

Other demonstrations were staged in the cities of Taez, Hudayda and Ibb, but all ended peacefully, witnesses said.

The fighter jet was downed by anti-aircraft guns near Arhab, 40 kilometres (26 miles) north of Sanaa, where armed tribesmen have been locked in combat with the elite Republican Guard, led by Saleh's son Ahmed, witnesses said.

"We saw the downed plane in flames on the ground," one witness said.

Tribesmen captured the pilot after he ejected when the plane crashed in the village of Beit Azar, tribal sources said.

The tribal area of Arhab has been targeted by heavy air strikes since a general and six other soldiers were killed Sunday in clashes between tribesmen and the Republican Guard.

General Abdullah al-Kulaibi, head of the 63rd brigade of the elite Republican Guard unit, died in the attack by tribesman opposed to Saleh's rule in the strategic town of Nihm, the defence ministry said.

Four of the attackers were killed during the assault on the military base, about 60 kilometres (40 miles) from the Yemeni capital, it said.

Tribal sources claimed on Monday that 33 troops were captured in the confrontation.

Meanwhile, three more gunmen were killed in overnight clashes with the guard, tribal sources said.

Nihm is one of several villages and towns that collectively make up the strategic northern gateway into Sanaa and is site of at least five Republican Guard bases.

The elite unit has so far prevented dissident General Ali Mohsen al-Ahmar, who now controls part of the capital, from calling in reinforcements from Yemen's northern provinces where parts of his division are deployed.

The tribesmen who carried out the assault on the military base late Sunday are allied with General Ahmar and have been battling government troops for control of the area.

Saleh, who is under international pressure to relinquish power and allow new elections, returned to the country last week, sparking violence in which scores have died.

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