The main Facebook page campaign, dubbed Women2Drive, says the action will keep going "until a royal decree allowing women to drive is issued".
A number of Saudi women have defied a ban on driving by getting behind the wheel in the conservative kingdom.
Friday's show of defiance came in response to calls on social networking sites to defy the ban that prohibits women from driving in the kingdom.
"We've just returned from the supermarket. My wife decided to start the day by driving to the store and back," said columnist Tawfiq Alsaif on his Twitter page on Friday.
"I took King Fahd Road [Riyadh artery] and then Olaya Street, along with my husband, I decided that the car for today is mine," local resident Maha al-Qahtani tweeted.
She also told the AFP news agency that driving is "a right for women that no law or religion bans ... I went out to get my right, so that it would be up to me to drive or not".
Her husband Mohammed al-Qahtani tweeted that she carried her necessary belongings "ready to go to prison without fear".
The en masse action was the first since November 1990, when a group of 47 Saudi women stunned men by driving around Riyadh in 15 cars before being arrested.
'Mutiny against male-only rules'
Activists had not appealed for mass protests in any specific sites, but had urged Saudi women to begin a mutiny against the male-only driving rules.
Women who had driving licences obtained abroad were urged to run their errands themselves without relying on male drivers.
"We want women from today to begin exercising their rights,'' Wajeha al-Huwaidar, a Saudi women's rights activist, said.
She had posted internet clips of herself driving in 2008.
"Today on the roads is just the opening in a long campaign," she said. "We will not go back.''
Friday is the climax of a two-month online campaign riding the winds of the so-called Arab spring, which has spread mass revolts across the region and toppled two regimes.