Turks are voting on constitutional amendments proposed by the government of Recep Tayyip Erdogan, the Turkish prime minister. Expected to be a tight contest, the referendum is seen as a crucial test for the governing party.
Voting started at 04:00 GMT on Sunday in the eastern provinces, and an hour later in the west. Opinion polls show that a small margin will decide the outcome of the referendum.
The results are expected several hours after polling stations close at 14:00 GMT.
The Konda Institute, which conducted a public survey before the voting began, revealed that the reforms, including controversial changes to the judiciary, would garner 56.8 per cent of vote.
However, the polls also showed that a good number of Turks remain undecided about the referendum, which is seen as an important gauge of support for Erdogan's Justice and Development (AKP) party before a parliamentary election next year.
Konda revealed that as much as 17.6 per cent of the electorate remained undecided ahead of the vote.
Tarhan Erdem, head of the Konda Institute, said voting in the referendum would mostly follow party loyalties.
"The lack of a strong party other than AKP is a major shortcoming of Turkish politics....falling opposition to the reforms is due to a decrease in support for the CHP," he told the Reuters news agency, referring to the opposition Republican People's Party.
Polls earlier this week suggested that the vote was too close to call, with one predicting a narrow defeat for the package and another suggesting it would pass by the flimsiest of margins.
Andrew Finkel, a journalist based in Istanbul, agreed that it was a tight call.
"Some people would like to see a sort of narrow win for the 'yes'...I think a lot of people see the package as being in Turkey's interest, but are reluctant to give an open endorsement to a government who has been in power for eight years," Finkel told Al Jazeera.
Some 50 million people are eligible to vote in the referendum that falls on the 30th anniversary of the 1980 military coup, which produced the current constitution, widely criticised for its oppressive spirit.
Erdogan has said that the reforms will bring the old constitution closer in line with those of the European Union, which Turkey seeks to join, but Finkel said this is an unlikely prospect.
"The current constitution was written in 1982, when Turkey was under martial law [and] it was promulgated by a military government and most Turks sense that this is not a document that will get them into the European Union.
"I think that people have really forgotten the issue of the constitution in the run up to this referendum debate [...] it has become very much a referendum, not on the constitutional package, but on the government itself," Finkel said.
Among the 26 articles comprising the package, changes are proposed in the way senior judges are selected.
Opposition parties, including the CHP secularists, argue that these will enable the government to exert more influence over the judiciary.
Defeat in the referendum would damage the ruling party's morale before a parliamentary election due by July 2011.
Source: Al Jazeera and agencies