Sri Lanka People’s party (SLPP) in a statement called upon the people to celebrate the victory peacefully.
The statement came as early trends indicated lead for Rajapaksa as he has got overwhelming support in majority-dominated areas of south.
Gotabaya has received around 48 percent of votes against 44 percent of his main rival Sajith Premadasa in nearly three million votes counted so far.
The latter has received wide support from minority areas in north and east. The counting of votes is continuing and final results are expected by evening.
Gotabaya has won most of the electorates in southern provinces including Hambantota, Galle and Matara, results of which have been declared so far.
However, Sajith has got over 80 percent votes from minority areas against less than 10 percent for his rival.
The trend shows sharp polarization along community lines, unprecedented in Sri Lankan history.
A candidate needs to poll over 50 percent of valid votes to be declared winner, else the second preference votes has to be counted.
Both candidates have campaigned mainly on the plank of national security and good governance.