It's almost sunset in Banda Aceh and the locals rush towards Ulee Lheue beach before the barrier goes down. The authorities close access to this popular spot after 6pm, to prevent promiscuity between unmarried couples.

Many families, couples, and groups of friends have arrived early, and enjoy a drink or corn on the cob at one of the food stalls lining the seaside. On the pier by the port, several couples brave the law by sitting closely together, sometimes holding hands. Luckily for them, the sharia police don't seem to be coming this evening. Islamic sharia law was adopted in 2001, a “gift” from Jakarta to quell separatist ambitions in this very religious part of Indonesia. A series of bylaws passed since impose Islamic dress code and forbid gambling, alcohol consumption and “seclusion” between unmarried couples.

Twenty-year old Leonie, chatting with two male friends while watching the sun go down on the sea, is one of the few women around not wearing a headscarf. She has been stopped by the sharia police before, at one of the checkpoints they regularly set up to “advise” people who do not abide by the dress code. Headscarves are mandatory, and wearing tight trousers or shirts, or for men, shorts, are a no-go. Leonie remembers the embarrassment of being reprimanded in public while having to wait with sharia police officers for her parents to come and pick her up. She hasn't learned her lesson though, and still refuses to cover her head. “I sometimes get comments on the street but I don't care, it's my problem.” What will she do if the sharia police come now? “Run,” she laughs.

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At the North-Western tip of the Indonesian archipelago, Aceh - Banda Aceh is the capital - is the only province in Muslim-majority Indonesia allowed to partly enforce sharia law, as part of a special autonomy status agreement that put an end to a 30 year conflict between Aceh separatists and the central government in Jakarta.

The sharia police have registered 13,000 offences between 2009 and 2013, the head of the unit, Zulkarnain, tells The Independent. He says “minor violations” of sharia law don't usually end up in the sharia court, even though the law states that three violations to the Islamic dress code requirements are punishable by caning. “This has never been enforced,” he says.

Serious violations, such as adultery and non-marital sex, are handled more severely. At the sharia police headquarters in Banda Aceh, a man and woman in their thirties are being questioned. The unmarried couple were caught having sex at a barber's shop. A suspicious neighbour had alerted the sharia police, a sign, says chief investigator Mazuki Ali, that the people support the sharia law enforcement. The investigation is ongoing, but Mazuki Ali says they face between six and nine strokes of the cane.

Although rarely enforced in Banda Aceh - the last public caning took place in 2007 - the punishment is common in other parts of the province. In the Eastern Aceh town of Langsa, the case of a 25-year-old widow caught last week with a married man by eight men, who raped her as a punishment, has outraged human right activists, especially after local authorities said she will still be caned for adultery. “She can't get caned, she's a victim, what she needs is support,” says Suraiya Kamaruzzaman, director of the women's group Flower Aceh. In the same town in 2010, three sharia police officers raped a 20 year old girl who they had arrested for riding a motorcycle with her boyfriend.

Activists monitoring the implementation of sharia law say abuses are on the rise, especially those committed by self-declared guardians of the law. Destika Gilang Lestari, director of the local human rights NGO, Kontras, says her organisation has recorded many cases of vigilante raids, in which young men often take it on themselves to punish offenders as it pleases them. “A couple was pushed into the river in Banda Aceh just a couple of weeks ago. People get beaten up, doused with sewage water, and they're often too scared to report it,” says Destika Gilang Lestari, who called the Langsa case a “barbaric act” and asked for the heaviest possible punishment. She says perpetrators too often remain unpunished, leading to a “misinterpretation” by vigilantes of what they're allowed to do.

While cases such as the Langsa one undoubtedly tarnish the image of the province, officials insist Islam is practiced in a “tolerant” and “moderate” manner in Aceh. “This is not Afghanistan,” says the head of the sharia police. In 2009, the local parliament added stoning to death as a punishment for adultery in a draft Islamic criminal code, but the then governor of the province rejected it.

Noting 2009 was an electoral year, the head of Aceh's Islamic sharia agency Syarizal Abbas suggests sharia is used as a political tool in parliament. “There seems to be a misinterpretation of what Islamic law is among Parliament members,” he says. “Islam in Acehnese society is very moderate. Implementation of sharia law in Aceh has to be done the soft way, it's more about education than punishment,” he adds.

Several media have reported that the recent adoption of a new Islamic criminal code was taking sharia law to a stricter level by imposing it on everyone, including non-Muslims, but officials strongly deny the claims. “This is absolutely not true,” says Syarizal Abbass. He insists the criminal code is only procedural, meaning it only defines proceedings and not the substance of the law. In any case he says, Aceh's Special autonomy law clearly specifies that the sharia only applies to Muslims.

For Muslims, who make up for 98 per cent of Acehnese, Islamic law enforcement is getting tougher. The Islamic criminal code adopted last February allows the sharia police to set up detention centres for suspected sharia offenders, and hold them for up to 20 days while their case is being investigated. Activists also say that sharia police raids on hotels and cafes, led by Banda Aceh's acting mayor - a woman - have intensified. Mazuki Ali says patrols do routine checks. “Patrols come at night, they check the hotel registry and if they suspect unmarried couples might be staying there, they check their rooms and IDs,” he says. If couples prove to be unmarried, they're taken to the police station.

Sitting at a cafe in central Banda Aceh, Davi, and two of his female friends, Rita and Ayu, share stories about friends arrested for wearing tight clothes or walking around with someone of the opposite sex. A sharia police truck carrying a dozen officers drives by. “They raided this cafe two weeks ago,” comments 23-year-old Davi. “It's a bit too much. We just hang out, we do nothing wrong.”