Yemeni forces backed by hundreds of armed tribesmen have launched an offensive to retake the southern town of Zinjibar, after months of fighting with Islamist fighters.

Security officials said Sunday that several militants have been killed in what they described as the fiercest fighting in the provincial capital since late May, when Zinjibar fell to Ansar al-Sharia, or Supporters of Sharia .

After weeks of pleas from the military's 25th brigade, which had been under siege for weeks, the government sent its first reinforcements to Zinjibar on Saturday. Local tribes agreed to join forces with the Yemeni military in its attempt to drive out Ansar al-Sharia, which the government says has links to al-Qaida.

Meanwhile, tens of thousands of people rallied Sunday in the country's two largest cities to demand the ouster of President Ali Abdullah Saleh on the 33rd anniversary of his autocratic rule.

Protesters in the southern city of Taiz waved black flags to mark the anniversary of Mr. Saleh's rise to power in 1978. Opposition activists also chanted anti-Saleh slogans in a main square of the capital, Sana'a, where they have been camping for months in an attempt to force him out of office.

Mr. Saleh has been receiving treatment in a Saudi hospital since suffering severe burns in a June 3 bomb attack on his presidential compound. The bombing also wounded his prime minister, the Sana'a governor and the chairman of Shura Council, a branch of Yemen's legislature.

Yemen's state news agency says the president visited the three wounded officials at the Saudi hospital on Sunday, and expressed happiness about what it calls the “constant progress in their health.” Mr. Saleh has rejected protesters' demands to step down immediately, insisting that he should lead a transition to a more democratic system of government.

It is not clear when he will return to Yemen, which has been led in his absence by his deputy, Vice President Abed Rabbo Mansour Hadi.

On Saturday, Yemeni opposition figures said they have formed a transitional council that will lead efforts to end Mr. Saleh's grip on power.

Opposition youth groups announced the 17-member council's formation in the capital, Sana'a. They also named their choices for some key leadership positions, including a former defense minister who would serve as the country's armed forces commander.

It is not clear what role the shadow government would play, and what its relationship would be to the formal political opposition.