People in Burkina Faso are voting in presidential and parliamentary dominated by concerns over security.

In Ouagadougou, Burkina Faso's capital, lines began to form as soon as voting opened on Sunday.

Around 6.5 million people are eligible to vote, but polls are canceled in about 1,500 villages hit by fighting.

Voters are to choose between 13 people standing for president, but the frontrunners are incumbent leader Roch Marc Christian Kabore and veteran politician Zephirin Diabre.

The exercise is taking place amidst a militant insurgency that's killed thousands and driven more than one million people from their homes.

First elected in 2015, Kabore is widely expected to win a second term. 

He's touted his achievements in building infrastructure and extending piped water, but he's been criticized for failing to stop the militant attacks.

The opposition accuses him of denying the army training and resources needed for a counter-insurgency campaign. 

'Plans for massive fraud'

Opposition parties in Burkina Faso said preparations were on for "massive fraud" in Sunday's presidential election and threatened to not recognize the vote results.

"It's clear that there is a huge operation orchestrated by those in power to carry out a massive fraud to legitimize," the re-election of the president, Zephirin Diabre, the main challenger to Kabore told reporters on Saturday.

"We will not accept results marred by irregularity," added Diabre, surrounded at a press conference by five of the other 11 opposition candidates, including Eddie Komboigo, the candidate of the party of former president Blaise Compaore.

Kabore can avoid a run-off election by winning more than 50 percent of the vote in Sunday's first round - as he did in the last election in 2015. The president's camp is promising a repeat performance.

However, his opponents say that will not be possible if the election is carried out fairly in the poor West African nation which has experienced multiple coups since gaining independence from France in 1960.

"It is absolutely inconceivable, having traveled the whole of Burkina Faso, to think of having a winner in the first round," said Diabre.

The security crisis has dominated the campaign and an undisclosed number of troops have been deployed for polling day in the landlocked West African country, one of the world's poorest.

The violence has forced one million people - five percent of the 20 million population - from their homes in the last two years, and at least 1,200 have been killed since 2015.

Most of the 12 opposition candidates running against Kabore have criticized the incumbent's failure to stem the bloodshed.