The death toll in a fight outside a polling station in Turkey has risen to three as voters cast their ballots in a historic referendum on expanding the president's powers.

The Anadolu news agency said a land feud may have been the reason for Sunday's deadly quarrel, while the private Dogan news agency reported it as caused by "differences in political opinion".

Two others were arrested after the incident.

Polls have now closed and counting has begun.

If the "yes" vote prevails in the referendum, the 18 constitutional changes will replace Turkey's parliamentary system of government with a presidential one, abolishing the office of the prime minister.

President Recep Tayyip Erdogan and his supporters say the "Turkish style" presidential system would bring stability and prosperity in a country rattled by last year's coup attempt and a series of devastating attacks by the Islamic State group and Kurdish militants.

But opponents fear the changes will lead to autocratic one-man rule, ensuring that Mr Erdogan, who has been accused of repressing rights and freedoms, could govern until 2029 with few checks and balances.

Mr Erdogan described the referendum as an opportunity for "change and transformation" as he voted in Istanbul, where bodyguards with automatic weapons stood guard outside the polling station.

"We need to make a decision that is beyond the ordinary," Mr Erdogan said, adding he hoped Turkish voters would make the "expected" decision.

Kemal Kilicdaroglu, the leader of Turkey's main opposition party and top "no" campaigner, called it a vote on Turkey's fate.

"We hope the results will be good and together we can have the opportunity to discuss Turkey's other fundamental problems," he said.

The official Anadolu news agency reported that military helicopters flew ballots and elections officers to some districts of Diyarbakir due to security reasons.

The proposed changes would grant the president powers to appoint ministers, senior government officials and half the members of Turkey's highest judicial body, as well as issue decrees and declare states of emergency.

It sets a limit of two five-year terms for presidents and also allows the president to remain at the helm of a political party. The changes would come into effect with the next general elections, scheduled for 2019.

Mr Erdogan, 63, first came to power in 2003 as prime minister and served in that role until becoming Turkey's first directly elected president in 2014.

He has long sought to expand the powers of the president. The result of Sunday's referendum will determine Turkey's long-term political future and will likely have lasting effects on its relations with the European Union and the world.