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 (Islamic law). 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 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. 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.