SANAA (AP) ― Renewed violence in the Yemeni capital killed at least 15 people on Thursday as forces loyal to the regime and its opponents shelled each other’s strategic positions from hills surrounding the city, medical and security official said.

The shelling over the city has terrified residents and emptied out city streets, already pockmarked by street battles between rival forces in different corners of the capital. A number of shops in a main boulevard in Sanaa were torched from earlier mortar shelling and oil spots covered the streets after electricity transformers also took a hit.

Smoke billowed from the opposite edges of the city, as two military officials said rival forces were caught in an exchange of artillery and mortar shelling from northern and southern hills at the edge of Sanaa. It was not clear what was hit by the shelling. The Republican Guards, forces loyal to Saleh and led by the son of President Ali Abdullah Saleh, have been in control of the south of Sanaa, while defecting military units led by Maj. Gen. Ali Mohsen al-Ahmar, a former Saleh aide who sided with the opposition, hold the city’s north.

Officials said six people were killed in central Sanaa when government forces shelled thousands gathered for a protest there with mortars and rocket propelled grenades. Snipers on rooftops also targeted the protesters at Change Square, the center of Yemen’s seven-month-old uprising, and adjacent streets.

Three bystanders were killed by a mortar shell in Sanaa’s northern Hassaba district, the officials said. The district is home to several of the tribal chiefs who switched sides in March to join the opposition against Saleh’s 33-year rule. The Interior Ministry later said four gunmen among supporters of Saleh were also killed. The rival side said one of its fighters was shot dead and 13 were wounded.