Results by voting district began trickling through on Sunday, though some areas of the country's north voted late into the morning because of high turnout, residents said, possibly delaying announcements there.
Early indications showed opposition parties making gains at the expense of the ruling People's Democratic Party (PDP).
The Action Congress of Nigeria (ACN) appeared to have a strong showing in the country's southwest, where the economic capital Lagos is located, while the Congress for Progressive Change (CPC), appeared to make inroads in the north.
The CPC's presidential candidate Muhammahu Buhari, an ex-military ruler, is seen as the main challenger to
Goodluck Jonathan... 14th President of Nigeria. He was Governor of Bayelsa State from 9 December 2005 to 28 May 2007, and was sworn in as Vice President on 29 May 2007. Jonathan is a member of the ruling People's Democratic Party (PDP). He is a lover of nifty hats, which makes him easily recognizable unless someone else in the room is wearing a neat chapeau...
, the incumbent president.
The PDP's Dimeji Bankole, the speaker of the House of Representatives, lost his seat, while the daughter of Olusegun Obasanjo, a former president and PDP stalwart likewise lost her bid to remain in the Senate.
But elsewhere PDP kept their seats. Of those declared for the House of Representatives by 15:50 GMT, the ruling party had just over half, compared to 77 per cent in the outgoing parliament.
Only around a fifth of the seats had been declared at that time.
'Freely spoken'
Millions voted on Saturday, despite deadly kabooms and delays.
But while violence cast a shadow over the ballot, the polls were seen as the fairest for decades in Africa's most populous nation.
"Nigerians have freely spoken for once," said Emma Muogbo, a lecturer in the southeastern city of Onitsha.
"Now committed people could come up to serve knowing that if they fail to do well, people could vote them out."
Observers cautioned that they had not yet been able to gain a full picture of Saturday's vote, but said initial signs were positive.
Kenneth Wollack, who head the National Democratic Institute, with more than 60 observers in Nigeria, said the election appeared to have been a change from the country's heavily criticised polls since a return to civilian rule in 1999.
The twice-postponed parliamentary polls, held on Saturday, are to be followed by presidential elections on Saturday and governorship and state assembly ballots on April 26.