LHOKSEUMAWE, INDONESIA—Authorities in Indonesia’s Aceh province are pressing ahead with a proposed Islamic law that would ban female passengers from straddling motorbikes despite reported opposition from the central government.

Aceh introduced a version of Shariah, or Islamic law, in 2009, after it gained autonomy from the government in a 2005 peace deal to end a long-running separatist war there. The Aceh laws regulate women’s dress and public morality, require shops and other places to close at prayer time, and are enforced by a special unit. Punishments can include public caning.

On Monday, authorities in northern Aceh distributed a notice to government offices and villages informing residents of the proposed law, which would apply to adolescent girls and women. It states that women are not allowed to straddle motorbikes unless it’s an “emergency,” and are not allowed to hold onto the driver.

Suaidi Yahya, mayor of the Aceh city of Lhokseumawe, said a ban was needed because the “curves of a woman’s body” are more visible when straddling a motorbike than when sitting sideways with legs dangling.

“Muslim women are not allowed to show their curves, it’s against Islamic teachings,” he said, declining to give details of what the punishment would be for violators.

Last week, Home Ministry officials told local media they would try and block the law because it was discriminatory.

While rare in the West, riding sidesaddle on a motorbike is common in much of Southeast Asia, particularly for women wearing skirts. There appears to have been no studies on which is safer, straddling or riding sidesaddle, though many women say they feel more secure and comfortable straddling.

Nurjanah Ismail, a lecturer on gender issues at the Ar Raniry Islamic Institute in Aceh’s capital, Banda Aceh, criticized the proposed law.

“There is no need to question this practice, let alone regulate it, because people do it for safety,” she said. “Women sitting in that way cannot be considered bad or in violation of Shariah. Islam is beautiful, so do not make it difficult.”

It is unclear how popular the Shariah provisions are with locals in Aceh, which while devout by Indonesian standards is a far cry from parts of Pakistan or the Middle East. Enforcement of the laws is patchy and mostly targets young men and women. Caning, when applied, typically is aimed at causing humiliation rather than pain.

Since 2005, many other regions in Indonesia have issued Shariah-inspired bylaws that ban such things as alcohol or tight clothing, alarming rights activists and others who value the country’s secular heritage. The government of President Susilo Bambang Yudhoyono, which relies on the support of Muslim political parties, has not spoken out against the laws, much less challenge them.