Last campaign for Guinea's opposition candidate Cellou Dalein Diallo

A huge crowd gathered in Conakry on Thursday to show their support to Guinea's main opposition leader and presidential candidate Cellou Dalein Diallo during his final rally ahead of the Sunday's election.

A sea of people, mostly without masks, took the streets of the capital of the country to see Diallo, who greeted them from the top of a truck.

The 68-year old-candidate has denounced President Alpha Conde's decision to run for a third term in office, calling it unconstitutional.

This will be the third face-off between Conde and Diallo, who first ran against each other in the country's historic 2010 election that came after more than a half-century of dictatorship.

For months, tens of thousands of people have taken to the streets in Guinea, and dozens have died in demonstrations opposing another term for Conde.

The current president insists he is following the will of the people by running in October 18 vote, after a public referendum approved it in March.

Diallo, of the Union of Democratic Forces of Guinea party, warned last month that Guinea has an unreliable voter registry, where more than three million people are not correctly registered and that the electoral commission has said it is unable to correct the shortcomings.

While the incumbent president previously defeated Diallo in both the 2010 and 2015 elections, many in Guinea say that Conde's popularity has sharply fallen as a result of his decision to seek a third term.

After surviving colonialism and dictatorship, many Guineans are fearful of the president's intentions.

Opponents now believe that Conde, 82, will use the new constitution to restart the clock on his term limits, potentially giving him another decade in power.

- 'Hate speech' -

Kabinet Fofana, a Guinean political scientist, warned there was a danger one of the candidates would not accept the outcome of the election.

"A major difficulty is the question of recognition, acceptance of the results of the ballot boxes," he said.

The outcome of Guinea's poll is likely to resonate further afield too, kicking off a string of elections this year across West Africa.

Activists are concerned that a win for Conde would bode ill for democratic norms in the region.

Aside from the third presidential term, Guinea's election campaign has been marked by fears of increased ethnic tensions in the diverse country.

For example, Conde -- who normally speaks French when addressing the nation -- last month told voters in the Malinke language that backing an opposition Malinke candidate amounted to voting for Diallo.

Politics in Guinea are mostly drawn along ethnic lines. President Conde's party is largely backed by Malinke people, and Diallo's UFDG by Fulani people, although both insist that they are pluralist.

Against a backdrop of concerns about ethnicity, representatives from the United Nations and African Union warned against "ethnic hate speech" in Guinea this month.

- 'To the cemetery' -

Diallo has accused Conde on the campaign trail of exploiting ethnic divisions -- a charge which he denies.

But the opposition candidate has also called into question the 82-year-old's ability to govern Guinea, urging him to "retire with dignity".

"He no longer has the physical and intellectual capacity to carry out this demanding function," Diallo told French broadcasters, of Conde.

Conde -- in seeming defiance of the criticism -- has crisscrossed Guinea at a fast clip over the past week, promising excited crowds to make the country "Africa's second (economic) power after Nigeria".

Despite huge mineral and fresh-water resources, Guinea remains a poor country, where about half of its population of some 13 million people live in poverty.

The president, for his part, has repeatedly criticised Diallo's tenure under Conte, an autocrat. He has also brushed off gibes about his age.

"Those who want to send me to the cemetery will go before me," he told party activists in the southern city of Kissidougou.

Ballots open in Guinea on Sunday morning; a second round is scheduled for November 24.