The People's Action Party's candidate Dr Koh Poh Koon and the Workers' Party's representative Ms Lee Li Lian are said to be fighting head-to-head in the race for the Punggol East single seat.
Sources told The Straits Times that the race remains too close to call, two hours after polls closed at 8pm and with counting of the ballots fully underway.
Signs are that the Reform Party's Mr Kenneth Jeyaretnam and Singapore Democratic Alliance's Mr Desmond Lim will lose their deposits.
Mr Lim has made a speech thanking his supporters, while Mr Jeyaretnam was last seen inside counting centre North Vista Secondary School.
There are 31,469 registered voters in the ward.
After the polls closed at 8pm, ballot boxes from the 10 polling centres were brought to North Vista Secondary School and the other counting centre, Rivervale Primary School.