* Parliamentary election first of three votes
* Signs of delay raise early questions over organisation
* Campaigning in some areas marred by violence
(Updates with Nigerians assemble for vote)
By Nick Tattersall
LAGOS, April 2 (Reuters) - Nigerians headed to polling stations on Saturday for parliamentary elections, the first in a series of ballots that will test whether Africa's most populous nation can break with a history of vote fraud and violence.
Ballot stuffing, intimidation and thuggery were so widespread in the last nationwide elections in 2007 that foreign observers questioned whether they reflected the will of the people, saying they fell far below international standards.
The electoral commission hopes a new voters' register, tighter polling procedures and better security will help stamp out fraud, but there has already been violence in several regions during campaigning.
Voters have to assemble at polling stations from 8 a.m. (0700 GMT) and register there before voting starts at 12.30 p.m. The aim is to prevent multiple voting. [ID:nLDE72U1JJ]
"I am anxiously waiting to cast my vote," said schoolteacher Balami Dalta, who reached the polling station before election officials in the village of Shaffa in the dusty northern Borno State. "I hope it will influence the outcome."
But the late arrival of ballot papers there and in some other centres raised early questions over organisation.
In the plush Ikoyi neighbourhood of Lagos, Nigeria's biggest city and commercial hub, policemen and polling officials sat at wooden benches under mango trees, eating breakfast as they waited for voters to arrive.
Land borders were closed ahead of Saturday's vote and only election officials, security forces and emergency staff were allowed to travel on the roads during voting hours.
Gunmen threw explosives into a police station in the central city of Bauchi late on Friday in an apparent attempt to cause panic, although many officers had already left for the day and there was no immediate confirmation of casualties.
Police in Delta state in the oil-producing Niger Delta, where there has been political violence in the past, said they had arrested two men in a minibus carrying AK-47 rifles, ammunition and a rocket launcher days ahead of the vote. <^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Graphic on elections: link.reuters.com/xet78r For more stories, background and analysis: [nLDE68H051] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^>
The build-up to the polls has risked exposing ethnic and religious fault lines 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 on the edge of the oil-producing Niger Delta, a series of killings in the northeast blamed on a radical Islamist sect, and sectarian clashes in central Nigeria 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.
Saturday's vote will be a litmus test for the presidential election a week later and the state governorship polls in two weeks' time, both of them ballots in which the stakes for the country as a whole are considerably higher.
Parliamentary elections are also fiercely contested by candidates eager for the spoils of the job -- a pay package whose allowances alone top $1 million a year. [ID:nLDE7300WJ]
Violence could reduce the turnout in the later polls.
President Goodluck Jonathan is seen as the front-runner in the presidential race on April 9, but the ruling People's Democratic Party (PDP) is expected to see its parliamentary majority reduced.
The PDP holds more than three-quarters of the 360 seats in the House of Representatives and of the 109 in the Senate.
U.N. Secretary General Ban Ki-Moon said he was encouraged by electoral reforms and successful elections would allow Nigeria "to continue to play its leading role in the promotion of regional peace, security and economic integration."
The National Emergency Management Agency has cancelled leave for its staff and identified a third of the country's 36 states as potential flashpoints during the elections.
Amnesty International said at least 20 people had been killed in political attacks and clashes in the past two weeks, and scores of cars and buildings had been burned. (For more Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/ ) (Additional reporting by Abdulwahab Muhammed in Bauchi, Austin Ekeinde in Port Harcourt, Camillus Eboh in Abuja; Writing by Nick Tattersall; Editing by Matthew Tostevin)