(Adds details of proposal, prime minister and official comments)
By Tuvan Gumrukcu and Ercan Gurses
ANKARA, Dec 10 (Reuters) - Turkey's ruling AK Party on Saturday submitted to parliament a package of constitutional reform proposals that would expand the president's powers, party officials said, in a move that could potentially see President Tayyip Erdogan rule until 2029.
Erdogan and his supporters have long argued that the country needs the strong leadership of an executive presidency, akin to the system in the United States or France, to avoid the fragile coalition governments that hampered its development in the past.
Opponents see the proposed change as a vehicle for Erdogan's ambition, and fear it will bring increased authoritarianism to a country already under fire from Western allies over its record on rights and freedoms, especially after widespread purges in the wake of a failed military coup in July.
"There will only be strong leaderships now," Prime Minister Binali Yildirim told reporters on Saturday, saying the changes meant the end of periods of coalition government.
"Parliament ... is being strengthened, while the presidency, in charge of the executive branch, is being restructured to end conflicts between branches."
The AKP wants to hold a referendum on the constitutional changes by next May and is seeking the backing of the nationalist MHP opposition to win approval for a national vote.
Any constitutional change needs the support of at least 330 deputies in the 550-seat assembly to go to a referendum. The AKP has 316 lawmakers eligible for voting, and the MHP 39.
Erdogan has turned a largely ceremonial presidency into a powerful platform at a time of domestic upheaval by drawing on his unrivalled popularity. The AKP is now seeking a strong executive presidency that while formalising his position's powers could avert any relapse into the fractious coalition governments of the 1990s.
Both Erdogan and the AKP have said they would put their proposed changes to the public even if they were to win 367 votes, which is theoretically enough to make constitutional changes without a referendum.
Speaking at a news conference following the submission of the proposals, senior officials Abdulhamit Gul from the AKP and Mehmet Parsak from the MHP outlined the specifics of the 21-article package, which Reuters has seen.
"This text ... carries the mission to the people of strengthening its leadership after July 15," Gul said, referring to the date of the failed military coup.
Presidential, parliamentary and local elections will all be held together in 2019 under the proposals, while the number of parliamentarians will be increased to 600 with each political party providing "substitute lawmakers" in the event of members no longer being able to take part in parliamentary activities.
If he were to win the presidential election, Erdogan could assume the executive presidency in 2019 and serve two five-year terms, keeping him in power until 2029.
In an unprecedented move, the proposals would allow the president to retain his ties to a political party, meaning Erdogan could resume his leadership of the AKP, which he founded.
The package would introduce criminal liability for the president, who previously was immune from all charges except treason. It also includes cutting the number of Constitutional Court members to 15 from the current 17 and abolishing military courts. The Gendarmerie would be removed from the national security council. (Editing by Hugh Lawson)
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