The election held on Saturday is crucial as the country seeks to recover from the worst economic crisis in its history and the resulting political upheaval.
The election, contested by 38 candidates, was largely a three-way race among Dissanayake, incumbent liberal President Ranil Wickremesinghe and opposition leader Sajith Premadasa.
According to tallies released by the Election Commission on Sunday, Dissanayake was leading with 47 per cent of total votes counted, followed by Premadasa with nearly 28 per cent and Wickremesinghe with 15 per cent.
Sri Lankan president Ranil Wickremesinghe has attracted just 15 per cent of the vote so far. (AP PHOTO)
The election was a virtual referendum on Wickremesinghe's leadership of a fragile recovery, including restructuring Sri Lanka's debt under an International Monetary Fund bailout program after it defaulted in 2022.
No major incidents were reported during the vote but authorities declared a countrywide curfew until midday Sunday as a precaution, police said.
There were 17 million eligible voters and final results are expected Sunday evening.
The government announced on Thursday that it passed the final hurdle in debt restructuring by reaching an agreement in principle with private bond holders.
At the time of its default, Sri Lanka's local and foreign debt totaled $US83 billion ($A122 billion). 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 with high taxes and living costs.
Both Premadasa and Dissanayake say they will renegotiate the IMF deal to make austerity measures more bearable.
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) that is crucial to maintaining stability.
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