– The Islamic State on Saturday released a video of what it said was its leader, Abu Bakr al-Baghdadi, leading Friday prayers in one of Mosul's oldest mosques, an event that not only displayed the group's military control over Iraq's second largest city but also served as a bold assertion of religious leadership.

In the video, Al-Baghdadi is referred to as the Caliph Ibrahim, a reference to the Islamic caliphate that the group announced a week ago in the areas it controls in Syria and Iraq and to which, it claims, all Muslims owe their allegiance.

"God gave your mujahedeen brothers victory after long years of jihad and patience … so they declared the caliphate and placed the caliph in charge," he said. "This is a duty on Muslims that has been lost for centuries."

A resident of Mosul said that Al-Baghdadi and his entourage arrived at the Great Nurridin Mosque without advance notice and that his security detail occupied the first row of worshipers, apparently wearing explosive vests.

The appearance of Al-Baghdadi at such a public gathering underscored the transition that the Islamic State, once a shadowy terrorist group most recently known as the Islamic State of Iraq and the Levant, is making to assert itself as a sovereign entity. Before the release of the video, the only known images of Al-Baghdadi were grainy photos released by the Iraqi government or displayed on a U.S. wanted poster.

The video, however, was an indication that Al-Baghdadi's days in the shadows are over. It was shot in high definition, making it seemingly easy for U.S. authorities to verify that he is the man they called Abu Dua and who was once held at the U.S. military's Camp Bucca detention center in Iraq. U.S. officials had no comment.

Charles Lister, an expert on extremist groups at the Brookings Institution's center in Doha, Qatar, said it was an assertion of authority. By appearing publicly as he did, Lister said, Al-Baghdadi was likely hoping to win "more expansive support from within the world's community of jihadist ideologues."