Since the campaign was launched in September, they have produced videos of women driving and put them on social networks.
They have helped protect the female drivers by forming packs of two or three cars to surround them and ward off potential harassment.
And some have simply ridden as passengers with the women as they run their daily errands.
"The stereotype is that there's a problem in Arab culture and that we are against women and that the West is on the side of women. This is totally rejected," said Abdullah al-Bilassi, a 23-year-old engineering student in Riyadh.
He says he is active in the campaign partly because he is tired of hearing that Saudi men and Islam are against allowing women to drive.
Though no laws ban women from driving in Saudi Arabia, the authorities do not issue them drivers' licenses. Many of the women who drove last weekend had licenses from abroad, activists said.
The tradition of banning women from driving is rooted in the kingdom's hard-line interpretation of Islam known as Wahabbism, with critics warning that women driving could unravel the very fabric of Saudi society.
But al-Bilassi said that driving is "a basic right. I don't know why they say society is against it."
Most of the men active in the campaign are in their 20s and 30s. They say their fathers and uncles often tell them frankly in male gatherings that it is not yet time to allow women to drive. Hard-line ultraconservative clerics warn that it could lead to "licentiousness."
Alaa Wardi of Riyadh, who says he is not involved in the campaign, has produced an online video called "No Woman, No Drive," using a Bob Marley song to mock comments by a prominent sheik who said driving can harm a woman's ovaries. It has had more than 8 million views since Saturday.
Rights activist and campaigner Ali al-Hattab from the coastal city of Jiddah said the monarchy uses the religious establishment's opposition to keep women from driving.
"The woman is a mother, daughter, wife and sister, so when the rights of a key member of society is affected, all the rights of society are affected," he said.
"They don't want women driving because it opens the door to a flood of other demands that will lead to calls for political reforms," he said. "It will evolve from a call for the rights of a segment of society to the rights of all society."
King Abdullah has gradually introduced reforms in Saudi Arabia, allowing women to sit on the national advisory council and permitting them to vote and run in municipal elections. But the stringent male guardian system is still in place, requiring women to obtain permission from a male relative to travel, get married, enroll in higher education or undergo surgery in some cases.