The Turkish parliament has begun debating proposed constitutional amendments that would hand president Recep Tayyip Erdogan sweeping executive powers.
Mr Erdogan, who has dominated Turkish politics for 14 years, has long-pushed the idea of giving the largely ceremonial presidency greater political powers, arguing that strong leadership would help the country grow.
The main opposition party fears that if approved, the reforms would concentrate too much power in Mr Erdogan's hands, turn the country into a de facto dictatorship and move Turkey away from democracy and its anchor in the West.
"They are trying to turn the democratic parliamentary regime into a totalitarian regime," Republican Peoples' Party leader Kemal Kilicdaroglu said.
The proposals would also allow Mr Erdogan to serve two more five-year terms.
Debate on the amendments is expected to last two weeks, and the reforms must clear two rounds of voting in parliament, gaining at least 330 of the 550 votes.
If the package is approved, the government will submit it to a voter referendum for final approval - possibly in the spring.
On Monday, police used pepper spray to disperse a group of politicians, lawyers and other protesters who tried to gather near an entrance to the parliament building to oppose the proposed changes.
Some roads leading to parliament were blocked in an apparent bid to prevent demonstrations.
The ruling party, founded by Mr Erdogan, is 14 votes short of the required 330 but has secured the backing of the country's nationalist party.
The changes would scrap the office of prime minister and make the president the head of the executive branch, as well as allowing him to appoint the government, dissolve parliament, propose budgets and declare states of emergency.
Currently, the prime minister leads the executive branch, while the president is mainly a figurehead with limited powers.
Other proposed amendments would increase the number of seats in the 550-member parliament to 600, reduce the minimum eligibility age for legislators from 25 to 18, and set parliamentary and presidential elections on the same day.
The debate comes at a difficult time for Turkey, which has been rocked by a wave of bombings, renewed conflict with Kurdish rebels in the south-east, a military offensive in Syria and a failed coup attempt.
The botched July 15 coup set the stage for a sweeping purge of state institutions that has alarmed rights groups and Western governments.
The government has argued a strong presidential system would reduce instability.