Even a year ago, it would have been hard to imagine -- Saudi women clad in skinny jeans and Harley-Davidson T-shirts, revving motorbikes at a Riyadh sports circuit.

But ahead of the historic lifting of a decades-long ban on female drivers on June 24, women gather weekly at the privately owned Bikers Skills Institute, to learn how to ride bikes.

"Biking has been a passion ever since I was a kid," said 31-year-old Noura, who declined to give her real name as she weighs public reactions in the ultra-conservative Islamic kingdom.

Overturning the world's only ban on female drivers, long a symbol of repression against women, is the most striking reform yet launched by powerful Crown Prince Mohammed bin Salman.

But it has been overshadowed by a wave of arrests of female activists -- including veteran campaigners who long resisted the ban.

None of the women at the floodlit motoring circuit wanted to talk about the crackdown, a deeply sensitive issue, focusing instead on securing a basic freedom long denied to them.

"I grew up watching my family riding bikes," Noura told AFP as she mounted a Yamaha Virago.

"Now I hope... to have enough skills to ride on the street." Next to her, revving a Suzuki, sat Leen Tinawi, a 19-year-old Saudi-born Jordanian.

For both women, biking is not just an adrenalin-fuelled passion, but also a form of empowerment.

"I can summarise the whole experience of riding a bike in one word -- freedom," Tinawi said.

Both bikers follow their Ukrainian instructor, 39-year-old Elena Bukaryeva, who rides a Harley-Davidson.

Most days the circuit is the domain of drag racers and bike enthusiasts -- all men. But since offering courses to women in February on the basics of bike riding, four female enthusiasts have enrolled, most of them Saudis, Bukaryeva said.

"They always wanted to learn how to ride a motorcycle. And now they are saying 'it's my time'," Bukaryeva told AFP.

She echoed a catchphrase printed on the institute's promotional material: "It's your turn to ride." Asked why more women had not enrolled for the course, which costs 1,500 riyals ($400, 340 euros), Bukaryeva said: "Maybe their families stop them." Tinawi echoed the sentiment, saying she faced strong reservations from her family. "My parents said: 'You on a bike? You are a girl. It's dangerous'," she told AFP.

In Saudi Arabia, taking the wheel has long been a man's prerogative. For decades, hardliners cited austere interpretations of Islam as they sought to justify the ban, with many asserting that allowing them to drive would promote promiscuity.

Many women fear they are still easy prey for conservatives in a nation where male "guardians" -- their fathers, husbands or other relatives -- can exercise arbitrary authority to make decisions on their behalf.

"Expect more accidents" because of women is a common refrain in an avalanche of sexist comments on Twitter.

The government has preemptively addressed concerns of abuse by outlawing sexual harassment with a prison term of up to five years and a maximum penalty of 300,000 riyals.

The most immediate practical worry for female motorists is the dress code. Inside the private institute, the bikers wear skinny jeans, with abrasion-proof knee pads wrapped outside -- but that is still unthinkable in public.

Body-shrouding abaya robes -- mandatory public wear for women -- are impractical while riding as their flowing hems could get caught up in the wheels. Many women also complain that female instructors are in short supply and that classes are expensive.

But topping all concerns is the crackdown on women activists -- while the kingdom trumpets women's rights.

Saudi Arabia this month said it detained 17 people for "undermining" the kingdom's security.

State-backed media published pictures of veteran driving activists, the word "traitor" stamped across them in red.

"It's a complete contradiction for the government to proclaim it is in favour of new freedoms for women and then target and detain women for demanding those freedoms," Samah Hadid, Amnesty International's Middle East director of campaigns, told AFP.

The arrests have unleashed a torrent of global criticism -- including from vocal supporters of Prince Mohammed's reform drive, such as Bernard Haykel, a professor at Princeton University.

Calling the crackdown "a mistake", he has urged the government to "apply due process and the rule of law" in handling jailed activists' cases.

Observers say the arrests seem calculated by the crown prince to placate clerics incensed by the modernisation drive and to send a clear signal that the pace of reform will be driven by him alone, not the activists.

Back at the institute, as the floodlights dimmed and the women bikers donned their abayas to leave, the crackdown was not a topic of discussion. "A climate of fear is now evident in Saudi Arabia," Hadid said.