The referendum on the political system may fall short of the turnout needed to make the results mandatory, pollster Alpha Research says.
The three-point national poll will be 100 000 votes below the threshold that will be binding, which, under the law, is at least 3.5 million for this particular referendum - or at least the number of people who cast ballots in the last parliamentary elections.
Alpha Research's projected activity is nearly 50.3%, but it does not reflect results for each of the questions which will be calculated separately.
Voters were asked on Sunday to have a say on three issues - the introduction of a majority voting system, compulsory voting, and a slash in state subsidies handed per vote to political parties - alongside the presidential election.
Regarless of the final figures, with more than two million people having voted, and an overwhelming support for all three issues, Parliament will have to hold a debate and vote on the questions. This applies to all all national polls where voter turnout was above 20%.
Official activitty statistics are due later on Monday.