With Islamic State of Iraq and the Levant fighters, flags and emblems everywhere, there is no mistaking who controls the north Syria city of Raqa.

Fighters from ISIL, the jihadist group that is now spearheading an offensive in Iraq, already regard the city that is strategically placed in the Euphrates valley as their "capital", activists say.

Since the jihadists first started moving into the city in 2012, they have been gradually imposing a brutal yet highly-organised system with all the trappings of a state, experts say.

And there is a very structured hierarchy -- Iraqi, Saudi and to a lesser extent Tunisian fighters and clerics call the shots, while Egyptian, European, Chechen and Syrian extremists are lower down in the ranks.

After rebels pushed forces loyal to President Bashar al-Assad out of Raqa last March, ISIL moved quickly to impose its iron-fisted rule.

Residents say the jihadists chased other rebel and Islamic groups out of town and kidnapped political and military rivals who dared to stay.

"In Raqa, ISIL has offices for everything you can imagine: health, education, security, Islamic aid, tribal relations management, and even an embassy of the emirate of Aleppo," according to Omar al-Huweidi, a writer and ISIL expert from Raqa pushed out by the group to Turkey.

"When jihadists first arrived in Raqa province in 2012, they were a group of 10 or 15. Today, ISIL controls every single aspect of life in Raqa," Huweidi told AFP.

ISIL has its roots in Al-Qaeda, and shares its ideology, but it split from Syria's official Al-Qaeda branch, the Al-Nusra Front, in late spring 2013.

"The difference is that Al-Nusra Front is postponing its imposition of hudud until it overthrows the regime, whereas ISIL has already started," said Huweidi, referring to Islamic punishments like amputations for thieves and execution for murderers.

For the group, Huweidi added, territorial control is also important, as suggested by their slogan "baqiya wa tatamaddad", which means that the Islamic state "is here to stay, and it's spreading."

ISIL has not publicised its structure, but activists have discerned its workings.

- Ruthless and highly intelligent -

A local described the official leader of Raqa, Emir Abu Luqman, as ruthless and highly intelligent.

Around him are foreign jihadists -- Iraqis and Saudis -- whom activists say are the main decision-makers.

ISIL's top clerics, said activist Abu Ibrahim, are Iraqi, Saudi and Tunisian.

"They espouse and promote the Al-Qaeda ideology, leading Friday prayers with speeches about general religious affairs, and more recently about ISIL's takeover of (Iraq's second city) Mosul," Abu Ibrahim told AFP via the Internet, speaking from Raqa.

Fighters, or "security men", play a more decisive role in ISIL's day-to-day affairs, according to Hadi Salameh, another activist working in the city, using a pseudonym to protect his identity.

The top men, he says, are Iraqi, including many who came from the ranks of Saddam Hussein's army, disbanded by the United States during the 2003 invasion.

"The way leaders are chosen depends on several factors, including whether they have done time in jails of the Syrian or other Arab regimes, or of the United States, in Iraq or Guantanamo," Salameh added.

Syrians are 'second-grade'

ISIL's top leader, Abu Bakr al-Baghdadi, is said to have studied Islamic law in his native Iraq, and spent four years in a US detention camp.

He was a part of the Al-Qaeda network that became the Islamic State of Iraq, ISIL's predecessor.

Aymenn Jawad al-Tamimi, a scholar and expert on Islamist movements, told AFP that little is known about Baghdadi, and that "the uncertainty shrouding him has boosted his personality cult".

YouTube is awash with Islamic-style anasheed (songs) singing the praises of Baghdadi, calling on potential recruits to swear loyalty (bayaa) to him.

Thousands of young Syrians, many of them uneducated, have flocked to ISIL, which they see as a radical, potent alternative to the poorly organised, ill-equipped rebel Free Syrian Army.

Part of the allure is the fact that the group is wealthy and well-armed.

It controls the strategic oil and gas fields in the eastern province of Deir Ezzor, which is contiguous to the Iraqi province Nineveh seized by ISIL in a sweeping offensive began on June 9.

Analysts say jihadist groups led by ISIL want to establish an Islamic state that would include the Iraqi provinces of Nineveh, Salaheddin, Diyala and Anbar, plus Raqa and Deir Ezzor.

Raqa, which serves as a conduit for all gas and oil supplies from Deir Ezzor, would be the natural capital.

ISIL has already set up tax collection systems across areas under its control.

"They pay their members salaries in dollars," said Salameh.

- Life is 'extremely difficult'

Activists describe life in Raqa as "extremely difficult" under ISIL.

"Wounded foreign jihadists get priority in hospitals. Syrians, even children, are second-grade," Salameh added.

For now, much to the Syrian opposition's dismay, residents of Raqa have little choice but to accept ISIL's terrifying rule.

"Key members of all the tribes have sworn loyalty to ISIL, out of fear rather than conviction," said Huweidi from Turkey.

Activist Salameh said: "I hate ISIL. But I have to admit, they are frighteningly well-organised, and I respect that about them."