Areej Khan, one of thousands of women who say they've had enough, drove for 25 minutes. Her campaign "We the Women" pasted the city of Jeddah with signs calling for the right to drive: "It's Not Only for Men" and "Driving Is Not Against My Religion" and "I Don't Like the Backseat."
She said that although she was scheduled to leave the country, she stayed to participate in the effort.
"I'm still sort of in shock," she told ABC News. "We actually passed by a car with guys in it, and no one bothered us. .... They were happy to see us."
Maha Qahtani drove in Riyadh this morning -- as her husband rode in the passenger seat.
"What kind of rule is this," she said to ABC News today. "Why no for us and yes for men? It's my right. It's my right and I have to have it."
'Saudi Women, Start Your Engines!'
Other women and men shared their experiences online:
"I'm on King Fahad Rd [from] AlGadeer Neighborhood 2 Takasosy Rd, I passed 2 police cars & they didn't stop me" Tweeted @Azizayousef.
"Take the wheel. Foot on the gas," said one Twitter message on the main site women2Drive.
Another person said: "Saudi women, start your engines!"
Sultan Al Quassemi, a writer and Arab affairs commentator, said the women-to-drive protest was already a success.
"It's a phenomenon that has been started by women, initiated by them [and] organized by them," Quassemi said. "This is a huge triumph."
Quassemi credited social media with helping women identify other women who were also willing to take up the driving challenge.
"It allowed women to upload proof that they have driven," Quassemi said. "It allowed women to share their stories instantaneously rather than over a period of days. It emancipated women. ... It gave them a voice that was instantaneous, that was direct, that was unfiltered."
Indeed, Khan said she didn't know whether people would take the challenge seriously until she saw a video of a woman in Riyadh driving after midnight.
"The groups we have on Facebook, everybody who has been out has uploaded their videos so it's sort of an online relationship with everyone," she said.
Activists did not appeal for mass protests but urged Saudi women to begin a growing backlash against the male-only driving rules supported by clerics. Today's campaign came a month after single mother Manal Al Sheikh was jailed and held for a week after posting video of herself driving.
Her arrest turbocharged the cause fueled by Facebook and publicized on Twitter -- and was supported by some husbands
"We've seen men support their women and say, 'I want my woman, my wife to be independent and that's why I want her to drive,'" Quassemi said.
Protesters say the ultraconservative kingdom's gender apartheid is not just unjust but hideously inefficient. In one example, Saudi women working at a factory in Riyadh can drive lorries but not drive themselves home. And for low-income women, paying a driver up to $400 a month is a burden they cannot afford.
"It adds to that social injustice that women feel already in the Middle East," Quassemi said. "You know, if they can ride camels 1,400 years ago, why can't they drive cars today?"
As Saudi women push back, the optimists say that change is ahead. "Now that I've done it, I'm worried that now I won't be able to stop," Khan said.
The Associated Press contributed to this article.