* New voter register seen as good start
* Bomb attacks new risk during campaigns
* Partiality of judiciary a concern
LAGOS, March 4 (Reuters) - Nigeria has completed a new 74-million-strong voter register it hopes will help ensure credible elections next month, but insecurity and the partiality of the courts remain challenges to a free and fair vote.
The Independent National Electoral Commission (INEC) said late on Thursday it had finalised a new electoral roll of 73,528,040 voters, a strong turnout in a population of around 150 million people, half of whom are under 18.
Previous elections in Africa's most populous nation were based on an electoral roll including names such us "Nelson Mandela" or "Michael Jackson" but omitting legitimate voters, skewing the results even before ballot boxes were stuffed or results sheets doctored.
INEC chairman Attahiru Jega has been praised by election observers, diplomats and politicians as more independent and credible than his predecessor, but overhauling the voter register was always going to be the easy part.
Preventing violence during campaigning and on polling day, ensuring results are not altered after collation and that voting booths are adequately protected, and making sure electoral disputes are settled impartially are bigger challenges.
"The manipulation of youth to perpetrate violence on behalf of political actors continues to be a significant threat to the integrity of the electoral process," the International Republican Institute (IRI), one of several outside bodies monitoring the vote, said after a pre-election visit to Nigeria.
At least four people were killed on Thursday by an explosive device thrown from a car at a ruling People's Democratic Party (PDP) rally in Suleja, near the capital Abuja, the latest act of violence in the build-up to the polls. [ID:nLDE72224L]
Suleja was not considered a particular flashpoint and the use of explosives to target civilians, until recently extremely rare in most of Nigeria, highlights the risk of indiscriminate violence even in areas considered relatively safe.
"It is utterly shocking, disturbing and disheartening to note that bombings are becoming part and parcel of the democratic process," former military ruler Ibrahim Babangida, a senior PDP member, said in a statement.
President Goodluck Jonathan condemned the attack as "most deplorable, callous, cowardly and wicked" and ordered the intelligence agencies to step up surveillance.
LITTLE CONFIDENCE IN COURTS
Localised violence has been a hallmark of Nigerian elections since independence from Britain in 1960 and three decades of military rule did little to develop a democratic culture.
Diplomats and security experts say the overall level of violence just a month away from voting day is lower than in the past, but bomb attacks are a new departure.
Those responsible for a car bomb in Abuja on New Year's Eve have still not been identified and there have been several attacks on opposition party offices and campaign rallies in Bayelsa state in the southern oil-producing Niger Delta.
"The use of crude IEDs (improvised explosive devices) at political venues is a very worrying development and such loss of life by homemade bombs at political gatherings in Nigeria is unusual," said a security contractor based in the country.
The IRI, whose pre-election mission to Nigeria was led by former Ghanaian President John Kufuor, also noted that the judiciary had laid itself open to accusations of partiality by assuming the role of adjudicator in electoral disputes.
"This has allowed for situations through which candidates can thwart the will of voters as well as placing the courts in a politicised environment detrimental to public confidence in (their) integrity," it said in its assessment.
It recommended that petitions should be independently reviewed before reaching the courts.
Five powerful ruling party state governors won a court ruling last week which could prevent them having to stand for re-election. [ID:nLDE71M20A]
As the incumbent, Jonathan is seen as the front-runner in the presidential race. But his main challenger, former military ruler Muhammadu Buhari, is expected to put in a strong showing in the mostly-Muslim north while the southwest, including the commercial capital Lagos, is also an opposition stronghold.
That could make it a close race. (For more Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/ ) (Additional reporting by Camillus Eboh and Felix Onuah in Abuja; editing by Tume Ahemba and Philippa Fletcher)