* Northern support key to chances of first round win
* Kano second most populous city after Lagos
* Northern elites fear marginalisation
KANO, Nigeria, March 16 (Reuters) - From Islamic police enforcing a ban on beer and prostitution to its centuries-old market and mosques, Nigeria's northern city of Kano feels like a different country to the pulsating southern sprawl of Lagos.
Its low-rise buildings and dusty tree-lined streets have more in common with the sleepy Sahelian cities of Niger or Chad than with Nigeria's commercial hub, a city built on hustle and home to some of Africa's largest companies and richest tycoons.
Securing support in this ancient city -- the second most populous after Lagos -- and other parts of Nigeria's Muslim north will be key if President Goodluck Jonathan, a southerner, is to clinch victory in the first round of elections next month.
As the incumbent, Jonathan is considered the front-runner, but his main rival, Muslim ex-military ruler Muhammadu Buhari, has strong grass roots support in the north and the opposition is hoping to force a run-off.
"What we are looking for is change. The only option is Buhari," said Sagir Haruna, a textile merchant in Kano's Kwari market, a labyrinth of narrow alleyways packed with stalls made of wooden lattice and corrugated iron roofs.
Cheers of "Buhari, Buhari" rang out from other stall holders as sirens wailed in the streets outside, signalling a security build-up ahead of Jonathan's arrival in the city, the latest stage of a campaign which has focused on the north this week.
Jonathan inherited power after the death of late northern President Umaru Yar'Adua last year and is seeking what would have been Yar'Adua's second term.
Some in the northern elite feel fate robbed them of what should have been another four years in power and there had been fears of an election campaign polarised around regional rivalries and a northern backlash if Jonathan wins.
But in the Kwari market debate, nobody mentioned the north-south issue, focusing instead on concerns common across the country such as the need for better electricity supply and security. Some stall holders voiced support for Jonathan.
"Jonathan has created job opportunities through recruiting in the civil defence corps, the immigration and police, and fuel shortages are a thing of the past," said Mutallah Ahmed, 35, shouting to be heard above the din.
Much is at stake in Kano and Nigeria's other northern cities. Jonathan must win at least 25 percent of the vote in two thirds of the country's 36 states to clinch victory. The north, along with opposition strongholds in the southwest, are seen as the most likely regions to prevent him succeeding.
Kano has just over 5 million registered voters, according to the electoral commission, second only to 6.1 million in Lagos.
The Muslim north long pulled the strings of political power. Four military rulers from the region led the country in the decade and a half before democracy was restored in 1999.
But since the end of military rule, the top brass of the security forces and upper echelons of the civil service have increasingly been staffed by southerners and some in the northern elite fear they are becoming marginalised.
The south has the oil reserves and sea ports and is home to the headquarters of Nigeria's banks. Rapid economic growth has created a generation of independent wealth no longer so reliant on the political patronage once dispensed by northern strongmen.
Buhari's reputation as a disciplinarian won respect from some, who lament the extent to which corruption has spread, and sits well with the conservatism of Kano, one of only four states where uniformed Islamic police are used to enforce sharia law.
Buhari ruled Nigeria between December 1983 and August 1985, an iron-fisted administration best remembered for its "War Against Indiscipline", a campaign against graft in which politicians were jailed and drug traffickers executed.
"Buhari has a clean mind. He is a very religious and Godly person," said Hajia Hadisa Mohammed, 85, stopping to join in the debate as she made her way through Kwari market.
Some observers fear victory in April for Jonathan could spark protest in parts of the north, inspired by popular uprisings in north Africa and the Middle East, particularly if the poll is deemed to have been less than free and fair.
But local religious leaders played down the threat.
"It is not about north or south, Christian or Muslim, we are beyond that," said Ibrahim Khalil, chairman of the Kano state council of Ulama, a grouping of Muslim elders.
"2011 will be one of the peaceful elections," he said, as the call to prayer sounded across the courtyard outside. (For full Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/ ) (Additional reporting by Mike Oboh; Writing by Nick Tattersall; editing by Joe Brock/Keith Weir)