Activists – inspired in part by the uprisings around the Arab world – have not appealed for mass protests in any specific sites.
But they urged Saudi women to begin a mutiny against the driving restrictions supported by clerics backing austere interpretations of Islam and enforced by powerful morality squads.
Encouragement boomed online. “Take the wheel. Foot on the gas,” said one Twitter message on the main site women2Drive.
Another urged: “Saudi women, start your engines!”
The defiance could bring difficult choices for the Western-backed Saudi authorities who so far have escaped major unrest from the Middle East turmoil.
Officials could either launch a crackdown on the women or give in to the demands at the risk of angering traditional-minded clerics and other groups opposing reforms.
It also could encourage wider reform bids by Saudi women, who have not been allowed to vote and must obtain permission from a male guardian to travel or take a job.
Security forces mostly stood by, activists reported, in an apparent effort to avoid clashes or international backlash. Some reported that women drove directly in front of police patrols.
No arrests or violence were immediately reported.
“We want women from today to begin exercising their rights,” said Wajeha al-Huwaidar, a Saudi women’s rights activist who posted internet clips of herself driving in 2008.
“Today on the roads is just the opening in a long campaign. We will not go back.”
The plan, she said, is for women who have obtained driving licences abroad to begin doing their daily errands and commuting on their own.
“We’ll keep it up until we get a royal decree removing the ban,” she told The Associated Press.
The campaign’s official start follows the 10-day detention last month of a 32-year-old woman, Manal al-Sherif, after she posted video of herself driving. She was released after reportedly signing a pledge that she would not drive again or speak publicly.
Her case, however, sparked an outcry from international rights groups and brought direct appeals to Saudi’s rulers to lift the driving ban on women - the only such countrywide rule in the world.
A protest supporter, Benjamin Joffe-Walt, said there were confirmed reports of at least several women in the driver’s seat in the capital, Riyadh.
Maha al-Qahtani, a computer specialist at the Ministry of Education, said she drove for 45 minutes around the city with her husband in the passenger seat. “I wanted to make a point,” she said in a telephone interview.
“I took it directly to the streets of the capital.”
Web message boards set up on Twitter and other social media carried unconfirmed reports that some women also got behind the wheel in the eastern city of Dammam and elsewhere.
A YouTube page urged supporters around the world to honk their car horns for the Saudi women.
But conservative forces also counter-attacked on the web.
One video – denouncing the “revolution of corruption” – featured patriotic songs and a sinister-looking black hand with red fingernails reaching for the Saudi flag.
On Facebook, a hard-line group had a message for Saudi women seeking the right to drive: “Dream on.”
Saudi Arabia has no written law barring women from driving – only fatwas, or religious edicts, by senior clerics following a strict brand of Islam known as Wahhabism.