What you need to know:
- Critics say that Conde, a former opposition leader, is drifting into authoritarianism by plotting to extend his grip on power.
- Diallo is now Guinea's main opposition leader, but he cut his teeth under authoritarian leader Lansana Conte, eventually rising to become prime minister.
Guinea's President Alpha Conde is seeking a controversial third term in Sunday's election, but the 82-year-old will have to stave off a stiff challenge from his longtime political foe, opposition leader Cellou Dalein Diallo.
Critics say that Conde, a former opposition leader who was once sentenced to death by an autocratic leader, is himself drifting into authoritarianism by plotting to extend his grip on power.
The president pushed through a revamped constitution in March that he said would modernise the country, but which opponents cast as a ploy to get around the West African country's two-term presidential limit.
Conde will face off against the slender and soft-spoken Diallo, who has been at the forefront of the protests.
Diallo is now Guinea's main opposition leader, but he cut his teeth under authoritarian leader Lansana Conte, eventually rising to become prime minister.
The campaign has been marked by fears of increased ethnic tensions in the nation of 13 million people, where politics are mostly drawn along ethnic lines.
Significantly, Diallo and Conde hail from different ethnic groups.
Conde's RPG party is largely backed by Malinke people, and Diallo's UFDG by Fulani people, although both insist that they are pluralist.
Conde has been married three times and has one son. A snappy dresser, he is also a skillful orator who can work a crowd.
Born in western Guinea in 1938, Conde headed to then-colonial power France at age 15, where he gained a string of diplomas in economics, law and sociology and then taught at the Sorbonne.
There, he led a French federation of African students and spurred opposition to the dictatorship of Guinea's first post-colonial leader, Ahmed Sekou Toure.
Sekou Toure had Conde sentenced to death in absentia in 1970.
Arrested and jailed
Conde returned to the country in 1991, seven years after the dictator's death, and contested presidential elections in 1993 and 1998.
But his continued activism was deemed a threat by then president Lansana Conte, who had him arrested just after 1998's election. Conde was subsequently jailed.
He then became Guinea's first democratically-elected president in 2010, after defeating Diallo in a runoff vote.
In power, Conde has sought to boost Guinea's lamentable electricity access and overhaul the military, all while combatting an Ebola epidemic that lasted from late 2013 until 2016.
Critics cast him as headstrong and authoritarian - and prone to angry outbursts.
He once mocked students who demanded he provide the tablet computers he had promised during election campaigning.
"You're like baby goats - 'tablets, tablets'," he said sarcastically, as he performed goat-like hops.
The president has also been dismissive about rights concerns.
Security forces have killed at least 50 protesters during anti-Conde protests since October last year, Amnesty International said this month, urging the government to investigate.
"I don't take Amnesty International seriously," Conde said in response. "They conduct inflammatory investigations".
Diallo is just as elegant a dresser as Conde. Fond of tailored suits in private, he often dons khaki jackets on the campaign trail.
Graceful and diplomatic, he has recently adopted the quirk of pointing at his watch in public - to show that Conde's time is running out.
Born to a large family in central Guinea in 1952, he attended both Koranic and French-language schools before studying management in the capital Conakry.
He then entered the civil service under Sekou Toure, before moving to the central bank under Lansana Conte.
A self-described "technocrat", Diallo held several ministerial portfolios under Conte before becoming prime minister in 2004.
He was dismissed in 2006, but to his critics, Diallo remains an apparatchik who symbolises the corruption of the Conte regime.
Diallo became head of the UFDG party in 2007, where he has remained ever since, seeing off hostility from the military junta that deposed Conte, and then leading the opposition to Conde.
Diallo has lost two elections against Conde, which he suspects are unfair. But he is convinced he will win on Sunday, pointing to Conde's "catastrophic record".
His decision to run again is controversial among some Guinean opposition supporters, who view his presidential bid as a legitimisation of Conde.
But Diallo says he is running to stop a third term, and for other supporters, his long experience in government stands in his favour.
Diallo pointed to Conde's advanced age several times during the campaign, encouraging the president to "retire with dignity".
"He no longer has the physical and intellectual capacity to carry out this demanding function," he recently told French broadcasters.
In defiance of jibes about his age, Conde has crisscrossed Guinea at a fast clip over the past week.
"Those who want to send me to the cemetery will go before me," he told party activists in the southern city of Kissidougou.