The frontrunners among the record 35 candidates -- who include two monks but just one woman -- are Sajith Premadasa, 52, son of assassinated president Ranasinghe Premadasa, and Gotabhaya Rajapaksa.
Gotabhaya, 70, is the younger brother of Mahinda Rajapaksa, president from 2005-15 and adored among the majority Sinhalese community for ending in 2009 the 37-year civil war with Tamil separatists in which 100,000 people died.
The horrific closing stages saw at least 40,000 Tamil civilians allegedly killed by government troops -- at a time when Gotabhaya was effectively in charge of the security forces.
He is also accused of running a "death squad" that bundled dozens of Tamils, political opponents, journalists and others into vans and dumping their bodies on the road -- something he denies.
One of its alleged victims was journalist Lasantha Wickrematunge, stabbed in the head in 2009 days before he was due to testify in a defamation case Gotabhaya had filed against his paper over corruption claims.
His daughter Ahimsa Wickrematunge said that she is "terrified" that under Gotabhaya "many brave police officers, prosecutors, witnesses, judges and journalists who have crossed his path... will find themselves on the firing line."
According to press watchdog Reporters Without Borders, in the "dark decade" of Mahinda's rule, at least 14 journalists "were murdered in connection with their work".
"Everyone needs to be afraid of what might happen if Gotabhaya becomes president, everyone," analyst Paikiasothy Saravanamuttu told AFP.
"For journalists, for any kind of dissent, it's going to be very, very tough."
A possible return of the Rajapaksas -- Gotabhaya would likely make Mahinda prime minister -- has also sparked fear among Sri Lanka's Muslims.
Relations between Muslims, who make up 10 percent of the population, and the Sinhalese have soured in the wake of attacks in April by homegrown Islamic extremists that killed 269 people.
In the days after the suicide bombings on three upscale hotels and three churches, hundreds of homes and shops owned by Muslims were trashed as mobs went on the rampage, with one person killed.
"This election, a lot of people are really worried," Reyyaz M. Salley, chairman of the Dewatagaha Jumma Mosque in downtown Colombo, told AFP.
What also concerns Western countries, as well as India, is that under Mahinda, strategically located Sri Lanka moved closer to China, even allowing two Chinese submarines to dock at Colombo in 2014.
China also helped shield Sri Lanka from international criticism of its human rights record under Mahinda in international forums such as the United Nations.
Beijing loaned and granted Sri Lanka billions of dollars for infrastructure projects under China's immense Belt and Road Initiative (BRI) spanning Asia and beyond. Mahinda says credit was unavailable elsewhere.
Sri Lanka was forced in 2017 to hand Beijing a 99-year lease on the port of Hambantota after being unable to service a $1.4-billion Chinese loan to build the harbour, highlighting for critics the debt dangers of the Belt and Road Initiative.
Many of the deals are also mired in corruption allegations. An airport built in the south has so far not been able to retain a single international airline.
At a time when the tourism sector has slumped following the April attacks, official figures show Sri Lanka will have to repay $5.9 billion this year.
Chinese investment "facilitated the mismanagement of the Rajapaksas," analyst Saravanamuttu said. "They spent and spent without giving any consideration to how it has to be paid back."