Most wanted man in Middle East lays down challenge not only to Baghdad and the west but also jilted mothership al-Qaida

For a man so mysterious that there are only two known photographs of him, it was a brazen public debut. The most wanted man in the Middle East, Abu Bakr al-Baghdadi is also one of the most elusive, an evanescent figure behind the Islamist insurrection sweeping the Syrian and Iraqi interior.

And yet according to jihadist websites at the weekend, here he was on video openly rallying the adepts of the new Islamic state he had just pronounced in the largest city that his fighters had taken. Clad in black robes that invoked a distant, almost mythical phase of Islamic history, Baghdadi gave a half-hour sermon during Friday prayers in Mosul and led worship inside one of the most important Islamic sites in Iraq in open defiance of the US intelligence officials who have put a $10m bounty on his head.

In doing so, he laid down a challenge not only to the authorities in Baghdad and the foreign powers that want stability in the country, but to the radical Islamist mothership from which the Isis movement broke – al-Qaida, and its current leader, Ayman al-Zawahiri.

Those present at the grand mosque in Mosul had no idea who would be preaching on Friday. But as the bearded figure made his entrance, he was introduced to them simply as "your new caliph Ibrahim".

"I am not better than you or more virtuous than you," Baghdadi says in the video. "If you see me on the right path, help me. If you see me on the wrong path, advise me and halt me. And obey me as far as I obey God."

An undated image of Abu Bakr al-Baghdadi distributed by the US state department. Photograph: AP

Baghdadi was born Ibrahim Awad al-Badari in 1971 near Samarra, a city 50 miles north of Baghdad. He took a master's degree and a PhD in Islamic studies at the University of Islamic Sciences in the Baghdad suburb of Adhamiya. When the US invaded Iraq in March 2003, the pious Baghdadi was still studying and was not thought to be connected to either al-Qaida or its local offshoot in the early years of resistance.

But by late 2005 he had been captured as a suspected mid-ranking figure in the anti-US Sunni insurgency. His jailers at Camp Bucca detention centre in southern Iraq have described him as inconspicuous.

After his release he was recruited to the military council of the Islamic State, acting as a key adviser to the then leader, Abu Omar al-Baghdadi. At the time the group was engaged in the intense sectarian war with Iraq's majority Shia and their militias.

"He wasn't the most impressive guy in the organisation," said another Islamic State member who spent time with Baghdadi in prison. "He wasn't even really a standout. He was a mid-ranking loyalist until he was freed."

Six months in detention was a major step in his transformation from devout Muslim to committed jihadist. However, it was another four years before he would assume the leadership of the movement, taking over from Abu Omar al-Baghdadi, who was killed in a US-led raid near Falluja.

He set about turning it from a local branch of al-Qaida into a distinct, independent force with a clear agenda: to re-establish a Sunni caliphate across Iraq and Syria.

Still, Baghdadi remained an unknown quantity until early last year when Isis started to make real inroads on the battlefields of northern Syria.

By the summer it had ousted a second jihadist group, Jabhat al-Nusra, and stamped its authority over the northern Aleppo countryside. It then set up a base in the eastern Syrian city of Raqaa, commandeering Syria's eastern oilfields and moving steadily eastwards towards Falluja and Ramadi in Iraq's Anbar province.

An Isis supporter in Raqqa, Syria. Photograph: Reuters

A senior Iraqi intelligence official told the Guardian that Baghdadi's determination to trump his rivals had led him to twice defy Zawahiri in the past year. The official said the most recent contact between the pair was on 4 June, days before Isis stormed Mosul and Tikrit.

"We intercepted letters between them, and the last one was Zawahiri complaining about Baghdadi writing to him to say he did not recognise his authority," the official said. "They were regularly swapping letters for several months. They were hand delivered and the turnaround was usually around 10 days."

The appearance of Baghdadi in the open in Mosul is deeply embarrassing for Iraqi officials who were caught hopelessly unawares when Isis militants began overrunning swaths of the Iraqi north last month.

The fragility of the state military has been exposed regularly ever since, with a 500-mile stretch of the border with Syria now under control of the insurgents, along with most of Anbar province and many of northern Iraq's military arsenals.

The besieged prime minister, Nouri al-Maliki, has constantly claimed to be making gains on the battlefields, but his forces are yet to claw back any of the losses to Isis.

And as the national military flounders, Shia militias are increasingly mobilising, with many members openly fighting out of sectarian motivations rather than a sense of patriotism.

Iraqi officials are scrambling to determine whether the video is genuine. The Guardian has spoken to two people who have met Baghdadi, both of whom said they were certain he was the man in the video.

Hisham al-Hashimi, a senior Iraqi researcher on Islamic militancy, said Baghdadi's choice of attire and language showed he was trying to liken himself to the earliest caliphs, especially those who ruled during the Abbasid period, a flourishing but brutal time in Islamic history.

Baghdadi had little new to say in his sermon, relying heavily on verses from the Qur'an and the words of other caliphs, in particular Abu Bakr Saddiq, the first caliph, who led the Islamic world after the death of the prophet Muhammad.

Scholars in Baghdad say the fact that Baghdadi can trace his lineage back to the Muslim prophet gives him significant leverage under sharia law, making it difficult for any senior cleric to contest his legitimacy as an Islamic leader.

"He is trying to seize a moment," said Hashimi. "He truly does believe he is a man worthy of historical comparison. He has all the all the criteria and the conditions. He belongs to the family tree."

In recent weeks Iraqi officials have attempted to suggest that Baghdadi is a deluded figure overcome by hubris. Some even suggested that he was the contemporary equivalent of a Jonestown cult leader, brainwashing followers into blindly following him in a nihilistic grab for power and influence.

However, the gains that Isis has made over two countries in such a short time suggest that its leadership is highly organised and efficient. "He is rational," said Hashimi.

"He thinks very clearly about what he is doing. He is deeply ideological and committed.

"He is also very determined to make himself into the one true ruler of Sunni Islam."

In Baghdad, the appearance of the video was met with alarm and curiosity in equal measure.

Azima Zahra, a resident of the capital, said Baghdadi's image would give her nightmares for weeks.

"He is what I thought he would be: a cold, mean and cruel man who thinks he is a prophet," she said. "He will lead us all to ruin."