The election, contested by 38 candidates, is largely a three-way race among incumbent liberal President Ranil Wickremesinghe, Marxist-leaning lawmaker Anura Kumara Dissanayake, and opposition leader Sajith Premadasa.
There are 17 million eligible voters, and final results are expected on Sunday.
The results will show whether Sri Lankans approve of Wickremesinghe's leadership of a fragile recovery, including restructuring Sri Lanka's debt under an International Monetary Fund (IMF) program after it defaulted in 2022.
The government announced on Thursday it passed the final hurdle in debt restructuring by reaching an agreement in principle with private bond holders.
Sri Lanka's local and foreign debt totalled $US83 billion ($A122 billion) at the time it defaulted, and the government says it has now restructured more than $US17 billion ($A25 billion).
Despite a significant improvement in key economic figures, Sri Lankans are struggling under high taxes and living costs.
Both Premadasa and Dissanayake say they will renegotiate the IMF deal to make austerity measures more bearable.Â
Wickremesinghe has warned that any move to alter the basics of the agreement could delay the release of a fourth tranche of nearly $US3 billion ($A4.4 billion) in assistance pledged by the IMF that's crucial to maintaining stability.
There are concerns that if a clear winner fails to emerge, the island nation could plunge into more instability.
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