A new public decency law will ban clothing deemed to "offend public tastes". (AFP)

Saudi Arabia has rendered toothless the once-feared religious police amid a liberalisation drive, but a planned "public decency" law is stoking controversy with some fearing a revival of morality policing.

Crown Prince Mohammed bin Salman has sought to shake off his country's ultra-conservative image with the reopening of cinemas, mixed-gender concerts and party-fuelled sporting extravaganzas, while vowing to take the kingdom towards moderate Islam.

The relaxed social norms in a kingdom seldom associated with fun in the past have been welcomed by many Saudis, two-thirds of whom are under 30, in the midst of painful economic reforms geared towards a post-oil future.

But Saudi Arabia now aims to police its citizens' behaviour with a new public decency law approved by cabinet in April, though it remains unclear when it will be enforced.

The law seeks to uphold Saudi "values and principles", banning in public clothing deemed to "offend public tastes" -- including men's shorts -- and graffiti that could be interpreted as "harmful", according to local media.

Violators reportedly risk facing a fine of up to 5,000 riyals ($1,333).

"The haia (religious police) is back without the beards," academic Sultan al-Amer said on Twitter.

The bearded enforcers of the religious police were long notorious for patrolling streets and malls to chastise women wearing bright nail polish and for rigidly imposing sex segregation, but their powers have been clipped in recent years.

"Effecting change is an art"

The law, widely perceived to be vague, has sparked public concern that it would be open to interpretation, leading to arbitrary penalties and, more light-heartedly, prompted humorous banter on social media.

The Arabic hashtag "shorts don't offend public morals" has gained traction alongside memes of men sweating it out on treadmills in loose-fitting traditional robes.

"It's Saudi Arabia meets Singapore," Kristin Diwan, of the Arab Gulf States Institute in Washington, told AFP.

"The Saudi leadership wants to undermine the Islamist basis of social power while still maintaining absolute political control and public order."

Pro-government Saudi media reported the law was meant to be implemented from May 25, with the interior ministry and tourism authority enforcing the rules.

But on May 27, state media said the law was yet to be enforced. It did not specify a new date.

"This (law) is an effort to balance the pressure from conservative elements of society that accuse the (government) of allowing things to go 'out of control'," said Ali Shihabi, founder of the pro-Saudi think-tank Arabia Foundation.

"Effecting social change is an art form -- you want to push as fast as possible without provoking a counter reaction. Not easy!"

Modernisation drive

Prince Mohammed, who has amassed powers unseen by previous rulers, has cut back the political role of the ultra-conservative religious establishment while promoting hyper-nationalism in a historic reordering of the Saudi state.

The kingdom's de facto ruler has projected himself as a modern-day reformer, while arresting several clerics -- including some perceived to be moderate -- and tightly controlling religious discourse as part of what observers call a broad centralisation of power.

Many other clerics appear to be toeing the official line, bestowing religious sanction on the prince's modernisation drive.

Prominent Salafi scholar Ayedh al-Qarni recently issued a televised apology for his previously well-known hardline interpretations of Islam, while throwing his weight behind the young prince.

While Saudi cleric Adil al-Kalbani, former imam of Mecca's Grand Mosque, has challenged the long-taboo subject of mixing the sexes by denouncing gender segregation in mosques as a "kind of phobia".

Still, the social transformation appears to meet resentment in conservative quarters, with many calling on the state to police public behaviour.

Last year, a viral video of a veiled Saudi woman and a man dancing and twirling on a busy street provoked fury, with many asking "Where are the religious police?".

Such calls could only grow louder, observers say, amid a previously unthinkable push to create a Dubai-style leisure and entertainment sector.

Testing the waters, self-styled religious scholars are openly advocating against shutting down businesses during prayer times and backing the opening of a temporary alcohol-free nightclub during a cultural festival in western Jeddah city.

"They (Saudis) are creating a broader realm for personal expression but only at the pleasure of the state," said Diwan.

"Graffiti must be authorised."