Turkish President Recep Tayyip Erdogan Friday made a final push for votes in a referendum on expanding his powers, as the authorities said they had foiled a possible jihadist plot against the poll.
Opinion polls -- usually treated with caution in Turkey -- have predicted a tight outcome on Sunday despite the considerable advantages of the 'Yes' campaign in both airtime and campaign resources.
The referendum will take place under a state of emergency that has been in place since last summer's failed coup, which has seen some 47,000 arrested in the biggest crackdown in Turkey's history.
Analysts regard the referendum as a crossroads in the modern history of the country that will affect not just the shape of its political system but also its relations with the West.
"April 16 will be a historic turning point," Erdogan told a rally in Konya, the Anatolian city seen as the heartland of conservative supporters who have benefited from his rule.
The referendum is taking place after a bloody year of terror attacks in Turkey blamed on jihadists and Kurdish militants.
Adding to security concerns, police detained five suspected Islamic State jihadists in Istanbul accused of planning a "sensational" attack targeting the weekend referendum.
Authorities on Tuesday detained another 19 suspected IS supporters in the Aegean city of Izmir, accused of planning to sabotage the vote.
In the latest issue of its Al-Naba magazine, IS called for attacks on polling stations in Turkey.
- Last-minute spat -
If the new system is passed, it will abolish the office of prime minister, enabling the president to centralise all state bureaucracy under his control and also to appoint cabinet ministers.
Supporters see the new system as an essential modernisation step for Turkey to streamline government but opponents fear it risks granting Erdogan authoritarian powers.
Erdogan in an earlier television interview expressed confidence the new presidential system would be approved, saying there were no longer undecided voters.
"'Yes' has gone up considerably, while 'No' has gone down," he said.
A poll by the Konda group showed 'Yes' ahead at 51.5 percent but the Sonar group has projected a 'No' vote of 51.2 percent, and with other polling companies producing different figures the outcome remains uncertain.
But while Erdogan has appeared confident and relaxed at his last campaign rallies, he reacted quickly to stamp out a last-minute spat with his main political ally in implementing the plan, the Nationalist Movement Party (MHP).
An advisor to Erdogan, Sukru Karatepe, suggested this week Turkey could form a federal system if 'Yes' won in a move that would be an anathema to the nationalists as it may imply the creation of a Kurdish region in the east.
MHP leader Devlet Bahceli quickly responded saying he would dismiss any advisor of his if they made similar remarks. MHP votes are crucial to the ruling Justice and Development Party (AKP) in pushing through the new system.
"I always have been and will always be the biggest supporter and biggest defender of Turkey's unitary structure," said Erdogan, adding that concerning a federal system "none of this is on our agenda".
The leader of the opposition Republican People's Party (CHP) Kemal Kilicdaroglu has called for a 'No' vote, arguing there was too much uncertainty over the consequences of the new system.
"We will together write a legend of democracy (on Sunday) because our questions have been left unanswered," he told reporters in Istanbul.
- UN raps Turkey -
Erdogan has raised hackles in the West throughout the campaign with his repeated denunciations of the European Union, which Turkey has sought to join for the last half century.
Ahead of the vote, UN experts accused Turkey of "massive violations" of the right to education and work, citing figures suggesting that since the emergency was declared, some 134,000 public servants had been dismissed.
The dismissals "cannot be justified by reference to Turkey’s longstanding international human rights obligations," said four UN special rapporteurs in a statement.
However, Turkey's foreign ministry rejected the "political statement", adding it would diminish the value of the UN mechanisms.
Campaigning is allowed until 6:00 pm (1500 GMT) on Saturday and voting in the country's east gets under way at 7:00 am (0400 GMT) on Sunday and an hour later elsewhere.