The Economic Community of West African States, or ECOWAS, said it would not recognize the results of the vote, which is expected to allow President Mamadou Tandja to tighten his grip on power and deepen the uranium-mining country’s constitutional crisis.
"The holding of the elections is a clear move by the authorities in Niger to further entrench the constitutional illegality currently prevailing in the country," said a spokesman for Nigerian President Umaru Yar’Adua, who is ECOWAS chairman.
Turnout for the election was patchy after the opposition called for a boycott of the vote.
Tandja’s second term in office was due to expire this year but he defied domestic and international pressure and extended his mandate for a further three years and increased his presidential powers at the expense of parliament’s.
"I hope for my people’s sake that those elected will be true patriots," the retired army colonel, who says he must stay to oversee lucrative infrastructure projects, said as he voted.
The White House urged ECOWAS to impose a full set of sanctions against the country.
"We urge ECOWAS to move towards the immediate imposition of full sanctions given President Tandja’s non-compliance with the ECOWAS request to suspend legislative elections," White House spokesman Robert Gibbs said in a statement.
"We urge President Tandja to resolve the political crisis in Niger through dialogue, rather than through self-serving actions that continue to erode Niger’s ten-year commitment to democracy and good governance."
Electoral commission president Moumouni Hamidou said voting had taken place with no major incidents reported by midday.
A Reuters witness in the capital, Niamey, said some polling booths had still not attracted any voters by mid-morning but turnout gradually increased by early afternoon.
"It is mainly women and young voters who have turned out," said Amadou Moussa, head of the polling station in the Kally South district of Niamey.
Tandja’s ruling party had promised to increase quotas for the representation of women in elected and governmental bodies, and to do more to tackle youth unemployment.
On Saturday, ECOWAS urged Tandja to delay the poll to allow talks with the opposition.
On Tuesday, it said it would cease economic and bilateral assistance. "The only assistance we can give to that country is the assistance that is aimed at the political crisis in Niger," Yar’Adua’s spokesman said.
Tandja’s plan to cement his position of power risks isolating the country internationally, said Hassoumi Massaoudou, spokesman for opposition party the PNDS.
"Niger will become a pariah state," he said.
Some candidates complained of irregularities, including a lack of ballots with their name at some rural booths.
"It is incomprehensible that the ballots of other candidates are available at some voting stations, but not ours," Abdoulaye Sanda Maiga, of the opposition Movement of Nigerien Patriots, or MPN, told local radio.
Polling ended at 7 p.m. (1800 GMT), with results due within three to five days.
French state-owned energy firm Areva, which has been digging uranium in Niger for decades, is spending 1.2 billion euros on a new mine, and China National Petroleum Corp signed a $5 billion deal there last June.
A referendum in August, condemned internationally and at home, eliminated many of the remaining checks on Tandja’s authority, abolished term limits, and gave him an initial three more years in power without facing an election.
The constitutional court declared that vote illegal, to which Tandja responded by abolishing the court and replacing its members with his own appointees.
Washington has denounced Tandja’s actions as undermining the rule of law, while the European Union has delayed aid payments.