About 60 million Nigerians were registered to vote
Partial results in Nigeria's first civilian-run elections since military rule ended four years ago give President Olusegun Obasanjo's People's Democratic Party a strong lead.
By early evening on Monday, 212 of the House of Representatives seats had been declared, with the PDP winning 119 of them.
Its nearest rival, All Nigeria People's Party, ANPP of the former military head of state Mohammadu Buhari, has 60 compared to 24 for the Alliance of Democracy.
In the Senate, the PDP has won 54 out of the seats declared so far. ANPP has so far won 32 whilst AD has 16 in the upper chamber.
At least 10 people were reported killed in violence related to the voting, but elections observers said the violence was not as bad as they had feared.
360 representatives' seats
109 senators' seats
260,000 security agents
120,000 polling stations
The commission needs to produce credible results before the presidential election on Saturday, in this key test for Nigeria's democracy, says the BBC's Dan Isaacs.
Nigeria has failed to transfer power from one elected government to another since independence in 1960.
In the south-west, stronghold of the opposition AD, the PDP has made significant gains.
But the PDP suffered setbacks in the north, losing key lower house seats to the ANPP, in Kano State - including the defeat of the speaker of the house, Ghali Umar Na'Abba.
The PDP was challenged by more than 30 other parties in Saturday's poll.
With the election results for most of Port Harcourt and the surrounding River States already announced, the ruling PDP won by a landslide, BBC News Online's Joseph Winter in Port Harcourt said.
But the size of the official turnout (95%) and the majority (about 90%) has sparked concern among opposition activists.
Local ANPP activists complained that the vote was marred by shooting, intimidation and the late arrival of voting materials, our correspondent added.
Across Nigeria there were scattered reports of election day violence and intimidation, especially in the south.
There are also concerns that many people who wanted to cast their votes on Saturday were unable to do so, with many polling stations opening late because of delays with ballot papers.
In the south and south-east, local leaders were demanding an extension to the voting and calling the election a sham, our correspondent says.
In the city of Warri, in the oil-producing Delta region, militant youths burnt down polling booths set up to allow a second day of voting.
These protesters are angered by what they see as the marginalisation of their own ethnic group in the whole political process.
At the results centre in Abuja, the Electoral Commissioner, Abel Guobadia, has played down these problems, saying that across the country, the vast majority of those who wanted to vote were able to do so.
Local and international observers had already expressed their concerns about the level of violence and intimidation in the run-up to the poll.
But preliminary statements issued by the Commonwealth and European Union observer groups say the election has generally been well conducted.
But expressed concern over logistical problems like the late opening of polling stations.