Police used teargas to disperse protesters rallying in support of a Sudanese woman facing 40 lashes for wearing trousers in public on Tuesday, a case that has become a public test of Sudan's indecency laws.

Lubna Hussein, a former journalist and U.N. press officer, was arrested with 12 other women during a party at a Khartoum restaurant in July and charged with being indecently dressed.

Women's groups have complained that the law gives no clear definition of indecent dress, leaving the decision of whether to arrest a woman up to individual police officers.

Dozens of men and ululating women gathered outside a Khartoum court room where Hussein appeared on Tuesday, carrying banners with the message "No return to the dark ages" and "Lubna's case is a case for all Sudanese women".

Speaking after the hearing, Hussein said the judge had adjourned her case until Sept. 7.

"They want to check with the U.N. whether I have immunity from prosecution. I don't know why they are doing this because I have already resigned from the United Nations. I think they just want to delay the case," she told Reuters.

Hussein said she resigned from her U.N. job last week to give up any legal immunity so that she could pursue the case to prove her innocence and challenge the decency law.

Journalists were allowed into the downtown court building, but a line of police prevented them from entering the room where the case was heard.

Hussein went on to greet her supporters, gathered on a traffic island, then led them on a march, blocking a busy road.

In a rare and short unauthorised rally, they flashed V-for- Victory signs and received supportive hoots from passing cars.

Armed riot police advanced towards the crowd, beating their shields with batons. Officers later fired teargas canisters into the air, forcing the crowd to disperse.

"We are against this law. It is against women, against Islam and against human rights," said Zainab Badradin, one of the women in the crowd.

"I want to send a message to the government to stop harassing women," said Nusayba Abdel Mahmoud, 20, who came to the rally wearing jeans. "This is what I wear. It is normal."

Indecency cases are not uncommon in Sudan, where there is a large cultural gap between the mostly Muslim and Arab-oriented north and the mainly Christian south.

Hussein has attracted attention by publicising her case, posing for photos in her loose green trousers and inviting journalists to campaign against dress codes sporadically imposed in the capital.

"Her main argument is that her clothes are decent and that she did not break the law," defence lawyer Nabil Adib Abdalla told Reuters shortly before the hearing.

"Failing that, we will ask for a stay of the proceedings to challenge the trial in the constitutional court ... We are saying the law is so widely drafted that it contravenes her basic right, her right to a fair trial," he added.

Hussein told reporters 10 of the women arrested with her pleaded guilty and were whipped the day afterwards.

U.N. Secretary-General Ban Ki-moon last week said he was "deeply concerned" by the case.