* Ruling party expected to see majority weaken
* Results slow arriving from remote areas
* Poll is litmus test ahead of presidential vote
By Matthew Tostevin and Camillus Eboh
ABUJA, April 10 (Reuters) - Election officials in Nigeria counted ballots from parliamentary polls through the night into Sunday, some arriving by horseback, in what they hope will be the nation's first credible vote in almost two decades.
Results starting to trickle in around Africa's most populous nation showed a strong performance by opposition parties, but in remote areas such as the northeastern state of Borno on the fringe of the Sahara, ballot papers were fetched by horse.
At a collation centre in Karo on the outskirts of Abuja, officials called out numbers above the hum of generators. Party representatives diligently noted the down the figures in exercise books and added them up.
Each of around 120,000 polling units counted and announced their results in front of voters on Saturday and observers were watching to see whether those figures were faithfully reflected at the centralised counts.
"Most of the numbers announced at the polling booths are the same as those read out in the collation centres, so the results so far are open, fair and justified," said Chinedu Michael, an observer from the Nigerian Committee on the Defence of Human Rights in the commercial capital Lagos.
President Goodluck Jonathan's ruling People's Democratic Party (PDP) is expected to see its parliamentary majority weaken, coming under strong pressure from the opposition Action Congress of Nigeria (ACN) in the southwest and the Congress for Progressive Change (CPC) in parts of the north.
"The ACN is leading in the Senatorial and House of Representatives race in most states in the southwest," Michael said in Lagos, based on feedback from his colleagues.
At the Karo counting centre it was clear that the CPC was edging out the PDP across most wards being collated there. There were no complaints or sign of cheating, though minor discrepancies were challenged in a laborious process.
Ruling party representative Theodore Ochei said he was disappointed the PDP had lost in his ward at the centre of Abuja, but not in the election itself.
"There is no instance of trying to rig, that's why the other parties are moving in," he said.
"Even if we lost as a party, we as politicians have more respect. Our leaders might sit up and begin to respect people's views."
Saturday's election, delayed by a week because of widespread logistical glitches, was the first in a cycle which includes presidential elections on April 16 and governorship polls in the country's 36 states on April 26.
It is seen as a litmus test of whether INEC can organise a smooth vote and make a break with a long history of polls discredited by ballot stuffing and thuggery.
The African giant has failed to hold a single vote deemed credible by observers since the end of military rule in 1999.
Most Nigerians say the closest the country came to a free election was in 1993, polls which were annulled by former military ruler Ibrahim Babangida, helping pave the way for another six years of military rule.
They are hopeful this time will be different.
"I am not tired because I am happy," said Nancy Godwin Bulus, a local observer from a women's group who had sat up through the night for the count in Karo.
"Nigerians are heading for democracy. We are advancing to maturity. We are orderly." (For full Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/ ) (Additional reporting by Nick Tattersall in Lagos and Ibrahim Mshelizza in Hawul; Writing by Nick Tattersall; editing by Joe Brock)