Turkey's parliament has given preliminary approval to a new constitution which will increase the powers of President Recep Tayyip Erdogan.
There will be a second round of voting later this week and, if approved, a referendum will follow.
Critics claim it amounts to a power grab by Mr Erdogan.
But the president says the changed system will resemble those in France and the United States.
The new constitution will allow the president to appoint and dismiss ministers, and it will abolish the post of prime minister for the first time in Turkey's history.
Instead there will be at least one vice-president.
The bill's final articles were passed late on Sunday, with the governing AK Party (AKP) gaining the three-fifths majority it needed.
Debates over the constitution changes have been heated.
Last week a fight broke out in parliament after the AKP clashed with Republican's People Party (CHP) members when an MP tried to film a voting session during a debate.
The CHP, the biggest opposition party, opposes the changes.
The pro-Kurdish Peoples' Democratic Party (HDP) boycotted the vote. Several of their MPs have been jailed on charges of supporting Kurdish militants, which, the HDP says, makes the vote controversial as they have no right to participate.
To secure its necessary majority, the AKP has relied on the support of the rightwing Nationalist Movement Party, the fourth largest in the legislature.
Turkey has been in a state of emergency since a failed coup in July. The status was extended after a series of attacks on the country, including a mass shooting in an Istanbul nightclub on New Year's Eve.
The constitutional amendments will give the president more scope for declaring an emergency.
Mr Erdogan, 62, came to power in 2002, a year after the AKP's formation.
He spent 11 years as Turkey's prime minister before becoming, in 2014, the country's first directly-elected president - a supposedly ceremonial role.