Turkish citizens voted on Sunday in a referendum which would give sweeping new powers to President Recep Tayyip Erdogan.
If the electorate votes “yes”, the role of prime minister would be abolished and executive power would be transferred to Mr Erdogan. Under the proposed changes, the President could in theory hold office until 2029.
He would also be able to appoint judges, pass decrees and have greater influence over the civil service.
The “Yes” campaign say the proposed changes would streamline Turkey’s political system, and point out the both the US and France have an executive presidency.
However, opponents fear the concentration of the power in the office of the Presidency will hasten the country’s drift towards authoritarianism.
Polls published earlier this week predicted a narrow majority for “yes” - between 51 and 52 per cent.
However, the referendum is clearly on a knife edge - with one poll stating that 8 per cent of Turks were still undecided.
In the run-up to the election, many Turkish commentators and activists have said the President’s crackdown on journalists has stifled debate around the referendum.
In the aftermath of the attempted coup against President Erdogan last year, at least 130 media organisations have been shut down.
There are currently around a hundred journalists in prison in Turkey, according to the international press freedom watchdog Reporters Without Borders (RSF).
Erol Önderoglu, RSF's representative in Turkey, told The Independent the government had been "intimidating Turkish civil society" in the lead-up to the referendum.
"Many journalists are behind bars, so the referendum will have been achieved, unfortunately, without any criticism of the government from a major TV station, for example.
"It goes without saying that state-owned TV channels and all the pro-government stations have, in a systematic manner, covered only the government’s perspective.
"In a referendum process... people should have access to different points of view and be informed properly. This is the opposite of what has happened in Turkey."
The government is also fighting a military campaign against the Kurdistan Workers' Party (PKK) in the south-east and is restricting press coverage of the campaign.
Thousands of Kurdish people displaced by conflict in Turkey may not be able to vote because of their lack of an address, according to Turkey’s Independent Election Monitoring Network.
Mr Erdogan became Turkish president in 2014, after more than a decade as prime minister. He has transformed the role, which was largely ceremonial before he took office.
The last polls in Turkey’s eastern provinces will close at 5pm (2pm GMT). Unofficial results are expected later on Sunday.