Turkish President Recep Tayyip Erdoğan has approved a constitutional reform bill that has been criticised as a power grab to strengthen his authoritarian rule.

If approved in a referendum, the bill would scrap the office of prime minister and give greater executive powers to the office of president.

The changes would give Erdoğan greater powers to appoint top officials and ministers, issue decrees and declare emergency rule.

He justified the changes as necessary after the instability that followed the unsuccessful coup attempt last July. However, critics say Erdoğan is trying to decapitate parliament.

"[Turkey] will take power from the parliament and give it to one man," Kemal Kılıçdaroğlu, head of the secular Republican People's Party (CHP), said in a speech.

"Can Turkey be dragged into an unpredictable adventure? Did we found the republic for this?" he said, reported AFP.

Erdoğan has now been in power 14 years, but if the bill is approved, he could remain in office till 2029. A date of 16 April has provisionally been set for the referendum.

Turkey initially approved the changes in parliament last December when Erdoğan's AK Party reached the 330 votes needed in the 550-seat assembly.

However the changes were deemed so controversial and divisive, politicians became embroiled in a mass brawl with shirt collars tugged and punches flying.

Protestors also gathered outside parliament and were met with tear gas and water cannons.

Defenders of the changes say those opposed will be able to voice their opinions in the referendum, which will pit Turkey's religious factions, which are loyal to Erdoğan, against the country's more secularist populace.

"People will have the final say," Deputy Prime Minister Numan Kurtulmuş said.

"Everyone – those who say 'yes' and those who say 'no' – will express their views," he said.

If approved, the changes would be the most drastic changes to Turkey's constitution since its creation in 1923.

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Turkish President Recep Tayyip Erdoğan has been in power for 14 years. Handout via Reuters