Several Saudi women have defied male-only driving rules to get behind the wheel, including one who managed a 45-minute trip through the nation's capital.
Activists - inspired in part by the uprisings around the Arab world - have not appealed for mass protests at any specific sites.
But they urged Saudi women to begin a mutiny against the driving restrictions that are supported by clerics backing austere interpretations of Islam and enforced by powerful morality squads.
Encouragement poured in via the internet. "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.
Wajeha al-Huwaidar, a Saudi women's rights activist who posted internet clips of herself driving in 2008, said: "We want women from today to begin exercising their rights. Today on the roads is just the opening in a long campaign. We will not go back."
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.