* Incumbent is favourite in presidential race
* Ruling party seen weaker in parliament, state votes
* Violence has marred campaigning in some areas
By Nick Tattersall
LAGOS, March 31 (Reuters) - Nigeria embarks on general elections on Saturday which could reshape its political landscape, weakening the decade-long dominance of its ruling party and breaking a history of rigged and violent polls.
Africa's most populous nation votes over three successive Saturdays for a new parliament, president and state governors respectively, all of them set to be fiercely contested races.
President Goodluck Jonathan is seen as the front-runner in the presidential race, but the ruling People's Democratic Party (PDP) is expected to see its parliamentary majority and some of its regional control weaken.
The build-up to the vote has risked exposing ethnic and religious faultlines in the country of 150 million people, roughly split between a Muslim north and Christian south but with sizeable minorities living in both regions.
There have been isolated bomb attacks on campaign rallies, riots in the state of Akwa Ibom on the edge of the oil-producing Niger Delta, a series of targeted killings in the remote northeast blamed on a radical Islamist sect, and sectarian clashes in the central "Middle Belt" in recent weeks.
But in other areas, observers have seen less evidence of politicians arming gangs to intimidate and harass voters than in the run-up to the last polls in 2007, and the electoral commission hopes tighter voting procedures will help stamp out the widespread ballot-stuffing and fraud of the past.
"This election is already fundamentally better than both previous elections," said Chris Newsom, an adviser to civil society group Stakeholder Democracy Network in the Niger Delta.
"It has some real contests. That is completely different to 2007, when real contests were very rare. The problem is real contests do not necessarily give you a result people think is fair, and that's the challenge." <^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Graphic on elections: link.reuters.com/xet78r For more stories, background and analysis: [nLDE68H051] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^>
The presidential race pits Jonathan, a Christian from the southern Niger Delta, against ex-military ruler Muhammadu Buhari, a Muslim with strong support in the northwest who seized power in a coup in 1983 and was overthrown two years later.
Former anti-corruption chief Nuhu Ribadu, whose Action Congress of Nigeria (ACN) party has a strong following in parts of the southwest, and Ibrahim Shekarau, governor of the northern state of Kano, are also vying for the presidency.
Jonathan must win an overall majority and at least 25 percent of the vote in two thirds of the country's 36 states to secure a first round victory. The opposition candidates are hoping their regional strengths can combine to force a run-off.
But they face an uphill struggle.
The PDP has won every presidential race since the end of military rule in 1999, holds more than three quarters of the seats in parliament and controls 27 of the country's 36 states.
"There are regional differences, the northwest, the southwest. In scenarios like that in Nigeria and other countries that typically favours the party in power," said Clifford Young of Washington-based research group Ipsos, whose poll last week put support for Jonathan at 60 percent and Buhari on 22 percent.
Buhari lost elections in 2003 and 2007, both marred by fraud, and has warned Nigerians will not tolerate another rigged vote. Few observers expect anything like the sort of protests seen in North Africa and the Middle East, but there could be a backlash in some parts if the polls are not perceived as fair.
"Perception, and how to manage expectations, will be key," said one Western diplomat.
The Independent National Electoral Commission (INEC) has registered more than 73 million voters on a new electoral roll, meant to be more credible than the previous one, which included false names such as "Nelson Mandela" and "Mike Tyson".
But security on the day of the votes will be the real test.
"I must confess we were amazed by the spate of violence which has erupted just a few weeks before the elections in some places, and we are mindful of this," INEC national commissioner Adedeji Soyebi told Reuters.
"But in many cases campaigns can be more violent than elections. Every polling booth will be manned by a security team ... The armed forces will not be at polling stations but they will be patrolling towns," he said.
Diplomats and observers are hoping they will be able to sign off on these elections as more credible than those of 2007. The last thing the international community needs, while it deals with conflicts in Libya and Ivory Coast, is another crisis.
But in the long term the challenge in Nigeria, where many people live on less than $2 a day and see politics as a game played by an elite with little bearing on their lives, will be how the political class makes itself relevant.
"The difficult business of governing democratically and contributing as good citizens of a democracy will continue long after the counting of votes in these important elections," said U.S. ambassador Terence McCulley.
"Democracy neither begins nor ends with elections." (For more Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/ ) (Additional reporting by Joe Brock in Abuja and Joe Penney in Lagos; Writing by Nick Tattersall; Editing by Giles Elgood)