'You know when you have a bird, and it's been in a cage all its life? When you open the cage door, it doesn't want to leave. It was that moment."

This is how Manal al-Sharif felt the first time she sat behind the wheel of a car in Saudi Arabia. The kingdom's taboo against women driving is only rarely broken. To hear her recount the experience is as thrilling as it must have been to sit in the passenger seat beside her. Well, almost.

Ms. Sharif says her moment of hesitation didn't last long. She pressed the gas pedal and in an instant her Cadillac SUV rolled forward. She spent the next hour circling the streets of Khobar, in the kingdom's eastern province, while a friend used an iPhone camera to record the journey.

It was May 2011, when much of the Middle East was convulsed with popular uprisings. Saudi women's-rights activists were stirring, too. They wondered if the Arab Spring would mark the end of the kingdom's ban on women driving. "Everyone around me was complaining about the ban but no one was doing anything," Ms. Sharif says. "The Arab Spring was happening all around us, so that inspired me to say, 'Let's call for an action instead of complaining.' "

The campaign started with a Facebook FB -0.04% page urging Saudi women to drive on a designated day, June 17, 2011. At first the page created great enthusiasm among activists. But then critics began injecting fear on and off the page. "The opponents were saying that 'there are wolves in the street, and they will rape you if you drive,' " Ms. Sharif recalls. "There needed to be one person who could break that wall, to make the others understand that 'it's OK, you can drive in the street. No one will rape you.' "

Ms. Sharif resolved to be that person, and the video she posted of herself driving around Khobar on May 17 became an instant YouTube hit. The news spread across Saudi media, too, and not all of the reactions were positive. Ms. Sharif received threatening phone calls and emails. "You have just opened the gates of hell on yourself," said an Islamist cleric. "Your grave is waiting," read one email.

Aramco, the national oil company where she was working as a computer-security consultant at the time, wasn't pleased, either. Ms. Sharif recalls that her manager scolded her: "What the hell are you doing?" In response, Ms. Sharif requested two weeks off. Before leaving on vacation, however, she wrote a message to her boss on an office blackboard: "2011. Mark this year. It will change every single rule that you know. You cannot lecture me about what I'm doing."

It was a stunning act of defiance in a country that takes very seriously the Quran's teaching: "Men are in charge of women." But less than a week after her first outing, Ms. Sharif got behind the wheel again, this time accompanied by her brother and his wife and child. "Where are the traffic police?" she recalls asking her brother as she put pedal to the metal once more. A rumor had been circulating that, since the driving ban isn't codified in law, the police wouldn't confront female drivers. "I wanted to test this," she says.