ISTANBUL - Campaigning for Turkey's crucial referendum on whether to expand presidential powers entered its final stretch Saturday, with supporters of both “yes” and “no” campaigns addressing flag-waving supporters in Turkey's two main cities of Istanbul and Ankara.

At stake is the future of Turkey's political system, with supporters saying the constitutional changes will herald a period of stability and prosperity, and detractors warning the reforms could lead to a system of autocratic one-man rule.

President Recep Tayyip Erdogan, who delivered a speech to a crowd in an Istanbul neighbourhood, has long championed the idea of changing Turkey's system of government from parliamentary to presidential. He is calling on his countrymen to vote on Sunday to approve 18 constitutional changes that would, among other things, abolish the office of the prime minister, handing all executive power to the president.

“The new constitution will bring stability and trust that is needed for our county to develop and grow,” Erdogan told supporters in Istanbul's Tuzla district. He also appealed to voters of other parties to approve the changes so that “Turkey can leap into the future.”

“Is it a 'yes' for one nation? Is it a 'yes' for one flag? Is it a 'yes' for one homeland? Is it a 'yes' for one state? Yes, yes, yes!” he said.

Erdogan said the proposed reforms could help counter a series of threats, including a failed military coup last year and a string of deadly bombings, some attributed to the Islamic State group. Fighting also resumed in 2015 between security forces and Kurdish rebels in the southeast of the country, after a hiatus of more than two years.

But critics argue Erdogan, who has been at the helm of Turkish government as prime minister or president since 2003, will simply cement his hold on power in a system that will have few checks and balances.

“Turkey is at a junction. We will make our decision tomorrow. Do we want a democratic parliamentary system or do we want a one-man regime?” Kemal Kilicdaroglu, leader of the main opposition Republican People's Party, asked supporters during a speech in the capital, Ankara.

The opposition has complained of a lopsided campaign, with Erdogan using the full resources of the state and the governing party to dominate the airwaves and blanket the country with pro-“yes” campaign posters. The “no” campaigners say they have recorded more than 100 incidents of intimidation, beatings and arbitrary detentions.

Erdogan has painted supporters of the “no” campaign as people bent on destabilizing the nation, accusing them of siding with terrorists blamed for the July 2016 attempted coup.

“Sunday will be a turning point in our struggle against terrorism,” Erdogan said.

The referendum comes as Turkey is still under a state of emergency declared after the failed coup. Some 100,000 people, including judges, lawyers, teachers, journalists and police, have been dismissed from their jobs. More than 40,000 people, including opposition pro-Kurdish legislators, have been arrested. Hundreds of news outlets and non-governmental organizations have been shut down.

“We want peace, freedom, democracy. We will have these with a 'no' vote tomorrow,” Pervin Buldan, a lawmaker from the opposition pro-Kurdish Peoples' Democratic Party, said at a rally in the predominantly Kurdish province of Diyarbakir.

Opinion polls indicate a tight race, and fierce campaigning was to continue until a ban comes into effect at 6 p.m. local time (1500 GMT) Saturday.

If approved in the referendum, the constitutional changes take effect with the next general elections, scheduled for 2019.