* Nigeria has sub-Saharan Africa's largest Muslim population
* Parts of north risk growing marginalisation
* April polls pit president against northern ex-ruler
By Nick Tattersall
KANO, Nigeria, March 18 (Reuters) - Standing on the dancefloor among shards of glass and splintered wood, Tony Baisie rues the day he agreed to help set up a nightclub in one of West Africa's oldest Islamic cities.
For more than 15 years this converted office on an industrial back street in Kano, northern Nigeria, was a thriving business. Customers -- Christian and Muslim -- would dance among its mirrored walls or shoot pool in the courtyard outside.
But three weeks ago, members of Hisbah -- a uniformed Islamic squad set up by Kano's state governor in 2003 to enforce sharia (Islamic law) -- raided the club, smashing tables and chairs, and seizing its drinks stocks and sound systems.
"They took me away and detained me overnight," Baisie said.
"Before they released me they made me sign an undertaking I would not sell alcohol or play music ever again in Kano."
Africa's most populous nation -- roughly divided into a Muslim north and Christian south, but with sizeable minorities living in both regions -- is full of paradox.
It is home to more Muslims than any other country in sub-Saharan Africa but is also the world's second biggest consumer of Guinness beer.
A secular government sits in Abuja in the centre of a fervently religious nation. Mega churches in Lagos to the south attract weekly congregations numbering tens of thousands, while the north traces its Islamic heritage back centuries to the trans-Saharan trading routes linking it to north Africa.
Around a dozen northern states have introduced sharia over the past decade, but it is practised to varying degrees and only four of them have enforcement squads like Kano's Hisbah, a force of around 9,000 men in green and black uniforms.
Sa'idu Ahmad Dukawa, their director-general, makes no apologies for what he says is a mission to purify the city.
"We are aware many non-Muslims in Kano are complaining bitterly that we are applying sharia on them because we are saying stop bringing alcohol to Kano, stop opening brothels. They feel it is an infringement of the rights of Christians," he said in his office, speaking in soft, measured tones.
"But I always argue that the fact we do not close churches, the fact we do not stop them from praying, the fact we do not force any one of them to Islam, I do not think there is any infringement. This is strictly social control."
Kano is generally accepted to be Nigeria's second most populous city after Lagos, although many of its residents insist on the accuracy of a 2006 census which put the state's population higher than its southern rival at 9.4 million people.
Its low-rise streets, lined by acacia and tamarind trees coated in a permanent layer of dust from the Sahara, are a far cry from the lagoon-side sprawl of Lagos, which spills chaotically across what were once mangrove swamps.
Kano's legions of motorcycle taxis, weaving their way around child beggars, are a familiar sight from other parts of Nigeria.
But in recent years they have started to carry predominantly male passengers, an outward sign of the city's growing Islamic conservatism. Women, their heads veiled, travel in yellow three-wheeled tricycles, a black curtain around the passenger compartment shielding them from sight.
At night much of the city is dead, apart from one street in the Christian neighbourhood of Sabon Gari. Plastic tables and chairs spill into the road around dimly-lit shacks. The Hisbah have agreed a pact with a few bar owners here and turn a blind eye, but the beer is kept behind curtains.
Some diplomats fear growing religious conservatism, hostility to Western values and pervasive poverty could allow radicals to gain a hold, particularly as they set the north at odds with the rest of Nigeria, which is increasingly liberalising as it opens up to ever greater foreign investment.
Southern Nigeria is home to the mainstay oil industry and the flourishing banking sector, while the textile industry and agriculture of the north are in decline. The upper echelons of the military and civil service have increasingly been staffed by southerners since the end of military rule 12 years ago.
"The impoverished North, with its rivalry between Muslims and Christians and its new, indigenous forms of radical Islam, has become increasingly alienated from the government in Abuja," wrote former U.S. ambassador John Campbell in a book published last year called "Nigeria: Dancing on the Brink".
"Without Abuja moves to counter this alienation, the North has the potential to provide a major impetus towards state failure," said Campbell, ambassador to Nigeria from 2004-2007.
Campaigning for elections next month, which pit southern President Goodluck Jonathan against northern former military ruler Muhammadu Buhari, has so far been largely free of the sort of rhetoric that seeks to whip up regional rivalries.
Kano state governor Ibrahim Shekarau, who set up the Hisbah, is also running in the presidential race but is a rank outsider.
Although Buhari is expected to win by a significant majority in Kano, campaign posters plastered on walls around the city are a patchwork of different candidates and parties, and plenty of ruling party supporters are vocal in debate on the street.
But some in the north fear another four years of rule by a southerner could further marginalise their region.
In the remote northeastern city of Maiduguri -- centre of the centuries-old Sultanate of Borno -- a radical Islamist sect has waged a campaign of violence against the state government.
Members of Boko Haram, which means "Western education is sinful", shot dead a leading governorship candidate in January and staged an uprising in 2009 in which hundreds died.
The leader of Hisbah said strict enforcement of sharia was a bulwark against such unrest in Kano and, in a thinly-veiled threat, warned against any new government undermining it.
"The average Muslim has a teaching which tells him that once a town is sanitized there should be no more violence," he said.
"The conscience of non-Muslims living in Kano is left to them. Either they sacrifice the alcohol business and prostitution business ... or they continue to fight it and bear the responsibility for the consequences," Dukawa said.
"The Ulama (Islamic scholars) will stop teaching that there is sharia, that Kano is sanitized, maybe some will go out of their way to tell people that non-Muslims living in Kano have killed sharia. And then let us wait and see the consequences." (For more Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/ ) (Editing by Diana Abdallah)