Abyan has already been declared capital of AQAP’s “Islamic Emirate,” over the weekend they overran Zinjibar.

SANA, Yemen — The Yemeni government ratcheted up its violent response to opponents on two fronts Monday, pounding a major coastal city with airstrikes aimed at dislodging Islamic militants, and smashing the country’s largest antigovernment demonstration in overnight clashes that killed more than a dozen protesters, according to witnesses reached by phone.

Residents in the coastal city of Zinjibar said warplanes attacked militant positions with repeated bombing runs beginning early Monday afternoon, a day after Islamist militants took control of the city, seizing banks and a central government compound. The army shelled the compound, which was also the target of many of the airstrikes, according to witnesses in the city.

As bombs fell in the restive south, security forces and plainclothes gunmen swept through a main square in the central city of Taiz, driving out thousands of antigovernment protesters and violently dismantling the country’s largest continuous sit-in.

It was unclear how many people had died in the Zinjibar fighting, which began on Friday: the city is facing a near total breakdown of services, residents said, with little medical services available, and no electricity or water. Hundreds have fled the city to Aden, the largest southern city, or to neighboring villages. Those who could not afford to leave have taken refuge in local mosques, residents said.