IRBIL, Iraq -- The Islamic State on Saturday released a video of what it said was its leader, Abu Bakr al-Baghdadi, directing 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 authority.
In the video, Mr. Baghdadi is referred to as the Caliph Ibrahim, a reference to the Islamic caliphate that the group announced a week ago in the area 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," Mr. Baghdadi said. "This is a duty on Muslims that has been lost for centuries."
A resident of Mosul reached by phone said that Mr. Baghdadi and his entourage arrived at the Great Nurridin Mosque, which dates to 1142, without advance notice and that his security detail occupied the first row of worshipers, apparently wearing explosive vests. The resident, who used to work at the mosque but cannot be otherwise identified for security reasons, said the group displayed the flag of the Islamic State.
In an apparent security precaution, cell phone service throughout the city was cut off for several hours Friday afternoon, other residents reported.
The appearance of Mr. 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 Syria, is making to assert itself as a sovereign entity. Before the release of the video, the only known images of Mr. Baghdadi were grainy photos released by the Iraqi government or displayed on a U.S. wanted poster offering a $10 million reward for his capture.
The video, however, was an indication that Mr. 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 on his wanted poster and whose "biometric" data presumably was collected during the five years or so when he was a prisoner at the U.S. military's Camp Bucca detention center in Iraq.
U.S. officials offered no comment.
Charles Lister, an expert on Iraqi and Syrian extremist groups at the Brookings Institution's center in Doha, Qatar, said the appearance was filled with historical references and was an important assertion of authority.
"That Baghdadi has chosen this time -- the first Friday after his declaration of the caliphate, during Ramadan, and in Mosul's Great Mosque -- is, of course, very symbolic," Mr. Lister said. Mr. Baghdadi's dress -- he was dressed in black and wore a black turban -- "very closely replicates that of the Abbasid caliphs," Mr. Lister said, giving him "at least to his support base, an image of understated authority."
The Abbasid caliphs ruled an area that stretched from present-day Iran to Libya from the eighth century to the 12th century. The various Abbasid capitals over the years included not just Baghdad, but also Samarra, the Iraqi city where Mr. Baghdadi was born in 1971, and Raqqa, the Syrian city that until the fall of Mosul June 10 was believed to be the Islamic State's administrative headquarters.
Mr. Lister said it would be hard to overstate the importance of Mr. Baghdadi's appearance in jihadist circles. "Put simply, one of the most wanted men on earth was able to travel into central Mosul and give a 30-minute sermon in the most venerated mosque in the largest city under control of the most notorious jihadist group of our time," he said. "Putting a face to a name gives the Islamic State a far improved chance of acquiring the pledges of allegiance that it now so desperately seeks."
In keeping with that interpretation, the Islamic State now refers to Mr. Baghdadi by his real name, Ibrahim Awwad al-Badri, or Caliph Ibrahim, an apparent effort to overcome initial skepticism expressed by mainstream Sunni Muslim scholars, who doubted the validity of the newly declared caliphate, in part, because Mr. Baghdadi had declared himself the leader of the world's 1.2 billion Muslims without ever appearing in public.