A Saudi Arabian woman who posted a video online of her driving her car is facing another 10 days in prison, according to reports from the kingdom.

Manal al-Sharif, a 32-year old mother who drove around the eastern city of Khobar last Saturday, had been expecting to be released on Friday after five days in jail on charges that her lawyer described as driving without a licence, provoking other women to do the same and provoking public opinion in Saudi Arabia. It is disputed by lawyers whether it is illegal for women to drive under national law but it is socially and religiously unacceptable in many quarters.

"The investigator needs another 10 days to complete his investigation," said Sharif's lawyer, Adnan al-Salah. "He will decide whether Manal is innocent and has to be released or he will refer her to the prosecution unit, a government organisation and they might refer her to a special prosecutor to deal with the case. I feel the fair and right thing would have been to release her on bail."

The extension of the investigation was interpreted as a show of defiance by the Saudi authorities in the face of growing domestic and international pressure to release Sharif.

"They have added 10 days to the investigation," said Waleed Abu Alkhair, a human rights activist, who knows Sharif. "This is very, very disappointing. The authorities want to show they are strong and don't care about any pressure."

The move is likely to fuel anger in Saudi Arabia and abroad at what is seen by some as an overzealous reaction to a woman driving her car. Sharif's case had already become a cause celebre among reformers and human rights activists in the highly conservative kingdom who are hoping to emulate aspects of the Arab spring freedom movements. King Abdullah is facing direct pressure from around 1,500 Saudis who have signed a petition calling for Sharif's immediate release and to clarify the law on whether women are allowed to drive.

The authorities were already braced for a planned mass-drive protest on 17 June, which was being organised by Sharif, an executive at the Saudi state oil giant Aramco. Since her arrest, several more videos showing women driving have been posted online in defiance of the state and conservative clerics.

The Guardian has learned that Saudi women are planning another mass drive within days to protest against a de facto driving ban in force across much of the kingdom. The event is being organised covertly with details circulated by email and text message in an attempt to catch the Saudi authorities off guard, human rights campaigners told the Guardian.

"There are many underground calls to take advantage of momentum and to do something right now," said one female organiser, under anonymity. "People are talking about women going out and driving and it is not just women who are supporting us, men are too."

On Thursday, the deputy interior minister, Prince Ahmed bin Abdulaziz, appeared to try to end debate about the issue by affirming that women cannot drive under a law decided in 1411, or in the Christian calender 1991, apparently contradicting previous remarks by senior members of the rulers saying it is a matter of social custom, not law.

"Women driving cars in Saudi Arabia has already been decided on in 1411 [and we decided] to not allow women driving," he said. "And this for us, the Ministry of the Interior, continues to stand." He added: "Our mission is to implement the system, but whether this action is right or wrong is not for us to say."

Salah insists it is not illegal for women to drive, even though only a handful in remote desert areas or in private compounds feel comfortable doing so and anyone with enough money hires a driver. As a result there are around 750,000 drivers in the kingdom, mostly migrant workers.

"You can't find any article in Saudi law relating to traffic that prevents women from driving," said Salah. "The government is taking this line because if they don't they may provoke the conservative part of society and they do not want to upset those people."

Sharif was first released after just five hours only to be rearrested in the early hours of Sunday when the authorities learned she had posted a video and encouraged other women to do the same. It showed her driving through the streets wearing a headscarf and black sunglasses telling the camera in Arabic: "If a husband has a heart attack what is a wife to do if there no one else around and she can't drive ... Not everyone is able to afford a driver. It's just too expensive for poorer families."

In an earlier interview, she said she was inspired to organise the Women2Drive movement by the experience of Bahia al-Mansour, a 20-year-old student at King Faisal University who started to struggle in her studies after difficulties in arranging transport.

"Every lady has something to do in the city, she'll just drive, do her business and come back," Sharif told the Dubai-based Gulf News. "So, it's as simple as that. People can't call it a demonstration, we're not going against the law, we're not going against anyone, we're not even demonstrating."

Her imprisonment sparked a furious reaction among some Saudis and social networking websites, Twitter and Facebook, buzzed with anger.

Eman al-Nafjan, a teacher and PhD student in Riyadh who writes a blog under the name Saudiwoman, told the Guardian that Saudi conservatives and the wealthy were determined to keep women from driving because it blocks anyone who cannot afford a driver from competing for jobs.

"Only the upper middle classes have drivers and that gives so much power to them," she said. "If you lift these obstacles then a lot of women will go out to work and society will change, they believe for the worse. Women will compete with men and they even believe it will cause more bastards to be left on the steps of mosques."

Sharif's arrest quickly became highly politicised. Her background as part of Saudi Arabia's minority Shia population was used by some to allege she was an agent of foreign powers, including Iran, aimed at destabilising the kingdom.

She is not alone in speaking up in favour of women driving. Najla Hariri, a 45-year old mother of five in Jeddah, started driving her own car earlier this month when her driver left and she had to take her son to school. Now she believes a challenge to the social stigma around women driving could be the first of many changes in the kingdom.

"Most of us have drivers, but sometimes we find ourselves in need," she said. "It is a very small thing, but we have the right to move from place to place. It is about independence. My driver left the job and I needed to take my son to school, so I took the car and went. That's all. I did it twice. The next day I went to the grocery shop to get some things. It shocked us all when Manal was arrested because she didn't break any law or breach any fatwa."

Samar Badawi, a human rights campaigner also from Jeddah, said she believes only a minority of men would object to women driving.

"It would change everything if women drove," she said. "Women would be able to go to hospital, take their children to school and do all this without a man. It would allow women to respect themselves if they drove their own cars. Maybe 15% of men would be offended, the rest would like women to drive. I know lots of women that drive, but Manal was the first to film it and put it on YouTube. This is why the government was angry."

Waleed Abu Alkhair, a human rights activist, who knows Al-Sharif, said his contacts in the Saudi court say they have to keep good relationships with the religious leaders.

"They feel sorry about what has happened to Manal but they don't want a bad situation with the clerics," he said.

Rumours circulated that she had broken down in custody and apologised, which was firmly denied by her lawyer.

"She didn't apologise for anything," Salah said. "She would have apologised if she had caused harm to anyone and she says she hasn't."

Hired male driver

Babal lives in a small hut at the front of Eman Al Nafjan's house in Riyadh. Whenever Eman wants to go somewhere the Bangladeshi man is always ready to drive her. Eman, the 32-year-old daughter of a military officer who works as a teacher and is also studying for a PhD in linguistics would rather Babal didn't have to. Babal is one of an estimated 750,000 drivers employed in Saudi Arabia, many from abroad, who ferry the richer women in the kingdom around because social custom, religious edict and possibly common law forbids them from driving themselves. He earns £196 per month and the family sometimes feed him, "but not three meals a day".

"Babal is there all the time," said Al Nafjan, the relatively affluent. "He has been with me for four years. He has a son, his wife is pregnant and he goes back to Bangladesh to see them once a year. I am very mobile but it is still inconvenient. Riyadh is a city of a million people and it's a one-and-half-hour trip to get to classes and I am there for three hours. There is no time to turn round and go back again so Babal waits. It is hot and I feel bad for the driver. He shouldn't have to. I should be able to drive myself."

• The following correction was printed in the Guardian's Corrections and clarifications column, Wednesday 15 June 2011. This article examined the background to the jailing of a Saudi woman, Manal al-Sharif, who had posted a video of herself driving a car. The piece described her as part of the country's minority Shia population: she is, in fact, Sunni. The piece also said that one of the charges against her, as described by her lawyer, was driving without a licence. We now understand she has two driving licences – one from the US state of Massachusetts and an international driving licence.