Two former presidents will face off against the incumbent in Somalia's long-delayed presidential vote on Sunday behind blast walls to protect lawmakers from Islamist attacks and meddling by factions within the security services.

President Mohamed Abdullahi Mohamed is among 39 candidates seeking the top post, to be chosen by parliamentarians in an airport hangar as the government's tenuous grip on the nation makes a popular vote impossible.

The winner will inherit a daunting list of challenges including the worst drought in 40 years, a violent conflict entering its fourth decade, clan feuds, and a power struggle between the government and federal member states.

"I am the most deserving person who can take Somalia to one man, one vote," President Mohamed, known as Farmaajo for his reputed love of Italian cheese, told lawmakers on Thursday.

Analysts say he may struggle to secure enough votes after allies failed to win senior roles in parliament last week.

At his inauguration in 2017, Mohamed promised to "finish" al Shabaab, an al Qaeda franchise, but the insurgents' attacks and extortion of households and businesses have continued.

Last year, Mohamed's failed bid to extend his term by two years sparked street battles in the capital Mogadishu as security forces split into factions, while his attempts to centralise power has irked regional leaders.

The election has been delayed repeatedly, putting a three-year $400-million International Monetary Fund (IMF) support programme at risk, as the president and prime minister traded accusations of trying to derail the process.

'SOMALIA CANNOT HAVE A DICTATOR'

Almost two-thirds of parliament, chosen by clan elders last month, are new, so lack a long voting record to help predict the outcome, political analysts said.

Despite the open field, the favourites include former presidents Sharif Sheikh Ahmed and Hassan Sheikh Mohamud.

Ahmed, a former Islamist, took over as head of a Western-backed transitional government in 2009.

Mohamud, a former academic whom donors accused of not doing enough to fight graft while in office, told parliament on Thursday he would reconcile clans and the federal states and hold a referendum on the constitution.

"Somalia cannot have a dictator leader who illegally clings to their seat," Mohamud said.

Close contenders and likely king-makers in the later rounds of voting are Puntland region president Said Abdulahi Deni and former Prime Minister Hassan Ali Khaire.

"I want to liberate our people from the poverty so that they can live with honour," Deni said.

Some residents of Mogadishu said they felt the election would be rigged and unlikely to change much at the top.

"We have seen such pseudo-elections before. I believe another liar will be elected," said Fardawsa Ahmed, a restaurant owner.

But local elder Aden Osman was more optimistic.

"Lawmakers used to elect the one who gave them the biggest hidden hand-shake (bribe). Now they may eat from all hands, but will choose a good man. That is good," he said.