The leader of the Islamic State group has made his first public appearance since proclaiming a caliphate, justifying the Sunni-led rebellion against the Iraqi government.

In a video posted on social media, the newly elected 'caliph', Abu Bakr al-Baghdadi, delivers a Friday sermon and leads prayers in the grand mosque of Mosul.

Baghdadi, who delivers a 15-minute-long sermon wearing a black turban and robe, spoke on the blessings of the Muslim fasting month of Ramadan, and the legitimacy of fighting in the path of God against oppression.

Quoting verses from the Quran, Baghdadi spoke on the need of establishing Islamic law and how God had helped "jihadists" establish a so-called caliphate.

"The establishment of a caliphate is an obligation," he said. "The religion cannot be in place unless the sharia is established."

The Islamic State, which has swept across much of northern and western Iraq, has tapped into the grievances of the country's Sunni community with Nouri al-Maliki, Iraq's prime minister, and his Shia-led government.

Baghdadi's purported descent from Prophet Muhammad's grandson was also mentioned in the video to comply with the requirement that a caliph be a member of the Prophet's Quraish tribe.

He also called on Muslims to obey him as long as he followed the "commands of God" and said he would not treat his subjects as other kings and rulers do.

"If you see that I am wrong, advise me and put me on the right track, and obey me as long as I obey God in you," he said.

Al Jazeera cannot independently verify the authenticity of the video.

Al Jazeera's Imran Khan, reporting from Baghdad, said it was a bold statement by Baghdadi, suggesting to the Iraqi government he was free to travel wherever he wanted in the territory of the self-declared caliphate.

Qaradawi's rejection

Yusuf al-Qaradawi, the prominent Sunni religious leader, on Saturday denounced Baghdadi's caliphate announcement as violating Islamic law.

"The declaration issued by the Islamic State is void under sharia and has dangerous consequences for the Sunnis in Iraq and for the revolt in Syria," he said. "We look forward to the coming caliphate."

Numerous figures from the Sunni community have dismissed the Islamic State's declaration of a caliphate.

Abu Muhammad al-Maqdisi, a Salafi leader from Jordan, called the group "deviant", while the pan-Islamist Hizb ut-Tahrir rejected the claims as "empty speech without substance".

Hizb ut-Tahrir said the Islamic State had no authority.

Since proclaiming the caliphate, the Islamic State has promised to sweep away state borders and redraw the map of the Middle East.

The group already dominates territory stretching from Aleppo in Syria to towns close to Baghdad.

Iraq government

Maliki, who has held the post since 2006, has rejected calls for him to step aside, with even some of his former allies blaming his failure to promote reconciliation for fuelling the discontent.

Iyad Allawi, the former Iraqi prime minister, on Saturday urged Maliki to give up his bid for a third term in power or risk the dismemberment of the country.

"If he stays on, I think there will be significant problems in the country and a lot of troubles. I believe that Iraq would go the route of dismemberment," he said.

Meanwhile, Maliki has removed the chief of the army's ground forces and the head of the federal police, according to Iraq's military spokesman.

Qassim al-Moussawi said Maliki retired Ali Ghaidan, the commander of the army's ground forces, and Mohsen al-Kaabi, the federal police chief.

Last month, Maliki sacked three generals who had been deployed in the north and ordered legal proceedings against them.