Authorities in Saudi Arabia are warning that women could be arrested if they defy a ban on driving by getting behind the wheel today as part of an online campaign.
Campaigners say as many as 17,000 people - men and women - have signed an online petition supporting women's rights to drive.
Saudi Arabia's Interior Ministry has contacted organisers of the campaign to end the ban on women driving and told them they will be punished if they go on defying the men-only road rules.
The women organising the campaign have been posting online footage of themselves driving in Saudi cities, and have called on Saudi women with foreign driving licences to get behind the wheel today.
The campaigners hope to take advantage of the ambiguous nature of the kingdom's ban on women driving, which is not explicitly enshrined in either the kingdom's Islamic Sharia law or its traffic code.
Saudi Arabia frequently earns bad international publicity over the issue, but any change in the effective ban on women driving might ignite the wrath of religious hardliners.
On Wednesday the Interior Ministry issued a statement reiterating that it was illegal for women to drive, but the authorities now appear to be stepping up their efforts to quash the campaign by individually contacting women involved.
"He said he was calling on behalf of (Interior Minister)Prince Mohammed bin Nayef and that I and any other woman should not drive and if we are caught we will be punished," said one of the campaign organisers who has decided not to drive in today's protest after she was contacted.
Another woman involved in the campaign, who also asked to remain anonymous, said she still planned to go ahead.
The Interior Ministry telephone calls follow a small protest by a group of conservative clerics demanding government action against the women.
One of them, Sheikh Nasser al-Omar, described the campaign as a "conspiracy".
Women who have driven in the past have often been charged with the relatively minor offence of driving without a valid Saudi licence, which are not issued to women in the kingdom.
But some have also been charged with more serious offences,such as disturbing public order or staging political protests,which are illegal in the absolute monarchy.
The campaigners say that by driving on Saturday they will not be staging a political protest, as they have not asked women to drive together in groups or to congregate in one place, even if they are in violation of traffic rules.
"The concerned authorities will enforce the law against all the violators with firmness and force," said Wednesday's Interior Ministry statement.
A Ministry spokesman said the ban applied to all women all the time.
He said a ban would also would apply to protests by groups opposed to women driving.
The prosecution service would decide whether to charge women drivers with traffic violations or more serious offences, he added.
Officials have often in the past said the driving ban is in place because Saudi society wants it there.
Supporters of Saturday's campaign say they want to show by driving without provoking public anger that society has changed.
They point to a recent move by some women in the kingdom's Shoura Council, a quasi-parliament appointed by the king to advise on policy, to challenge the ban, and to Saudi newspaper columns that argue women should be able to drive.
"The government now is in an odd position. They aren't against women driving and yet they're preventing women driving.It's very awkward to be in this position," said Khalidal-Dakhil, a Saudi political science professor and columnist for the pan-Arab daily al-Hayat.