A voter casts her ballot at a polling station during the country's presidential and legislative elections in Niamey, Niger, February 21, 2016. [Photo/Agencies]

NIAMEY - Niger closed its land borders and ramped up security on Sunday for an election in which President Mahamadou Issoufou is running for a second term promising to crush Islamist militants and reduce the country's deep poverty.

Security forces patrolled cities and villages in case of unrest or militant attacks. Some voters said they had never experienced such a tense election.

Unidentified armed men attacked two electoral commission vehicles in a rural area about 100 km (60 miles) northwest of the capital, according to security sources, but there were few other reports of trouble.

"Niger needs strong democratic institutions. I hope that the presidential and legislative elections will permit us to reinforce our institutions," Issoufou said when he cast his ballot at city hall in the capital Niamey.

He faces 14 candidates including Seyni Oumaru, leader of an opposition coalition. Critics say Issoufou has used political repression in the run-up to the vote, arresting opposition supporters and jailing opposition leader Hama Amadou over charges related to a baby-trafficking ring.

"These are not free and fair elections. We have one presidential candidate in prison who has not been able to campaign. ... The president has manipulated the electorate and used repression," said Amadou Saidou, a voter in Niamey.

The government says it respects the law and calls such criticisms politically motivated.

Voting ended at 7 p.m. (1800 GMT) after a day of steady turnout in most areas but polls will reopen from 8 a.m. to 7 p.m. on Monday in areas where voting could not take place on Sunday, the electoral commission said.

It gave no details but local media reported problems in Tahoua region in the northeast, Zinder in the east, Diffa in the southeast and Tillaberi in the west. The country has eight regions in total and results are not expected before Tuesday.

Opposition spokesman Ousseini Salatou said on a private television station, Tenere, that the election had been badly organized and he had witnessed cases of voting card fraud.

Niger produces uranium and oil but is ranked last in the U.N.'s Human Development Index and has one of the world's highest fertility rates. The country ranks 114 out of 142 in the 2015 prosperity index run by the UK-based Legatum Institute.

Islamist militant group Boko Haram, which is based in neighbouring Nigeria, has staged a series of attacks in Niger in recent months, forcing authorities to declare a state of emergency in the southeastern region of Diffa.

But Niger prides itself on being peaceful relative to its neighbours Nigeria, Libya and Mali.

Issoufou, born in 1951, won an election in 2011, a year after a coup. Under election rules, a run-off will be held if no candidate secures an outright victory on Sunday.

His challengers include Amadou, 2011 second-place finisher Oumaru and ex-president Mahamane Ousmane. Around 5,200 candidates also vie for 171 legislative seats on Sunday.