KANO, Nigeria (AP) — An explosion tore through the central mosque in Nigeria's second-largest city on Friday, and officials feared the casualty toll would be high.

Capt. Ikechukwu Eze said the Friday blast occurred at the main mosque in the city of Kano. Hundreds had gathered to listen to a sermon in a region terrorized by attacks from the militant group Boko Haram.

Witnesses say heavy smoke could be seen billowing in the sky from a long distance away. Immediately after the blasts, hundreds of angry youth took to the streets in riots, throwing stones, brandishes sticks and shouting at security officials. The palace of the Emir of Kano is near the central mosque. Palace officials told AP that the Emir, one of the highest ranking Islamic figures in Nigeria, is currently out of the country.

Eze did not immediately provide a precise death toll.

The militant group Boko Haram has carried out numerous such attacks in northern Nigeria and Kano has been repeatedly hit. In September, two suicide bombers killed at least 15 students at a government college. In July, five suicide bombings were carried out over the course of a week. More than 1,500 have been killed this year in the insurgency.

Meanwhile, a police anti-bomb squad defused six bombs planted near a mosque and a market in Maiduguri on Friday, according to Borno state Police spokesman Gideon Jubrin.

Fears are running high in Maiduguri after two female suicide bombers detonated explosives on Tuesday at a commercial center. At least 70 people were killed.