SEOUL - South Korean President Moon Jae-in on Monday proposed weakening the powers of his office and lowering the voting age in a package of constitutional reforms, while allowing the head of state to be re-elected.

South Korea is a vibrant democracy but its executive presidency is extremely powerful, giving rise to a winner-takes-all politics which critics say enables corruption while reducing representation for opposition voices.

In last year's election Moon campaigned on a promise to reform the constitution for the first time in three decades.

The vote was a by-election to choose a successor to his ousted predecessor Park Geun-hye, toppled over a wide-ranging corruption scandal that exposed shady links between big business and politics. Prosecutors are now seeking a 30 year jail sentence for her, and her own predecessor Lee Myung-bak was arrested last week in a separate inquiry.

Moon's plan has to be approved by parliament before being put to a referendum in June, and its centrepiece measure would see the country's single five-year presidency be reduced to a four year term, with one opportunity to stand for re-election.

South Korea brought in term limits after the assassination of the late dictator Park Chung-hee, Park Geun-hye's father, who ruled from 1961 to 1979 and revised the constitution to allow him to rule indefinitely.

He also made several constitutional changes to strengthen presidential authority - many of which remained in place decades later.

Supporters say two four-year terms would encourage longer-range thinking in the presidential Blue House, while driving incumbents to the centre ground to preserve their chances of re-election.

The bill also includes lowering the voting age from 19 to 18 and giving parliament oversight of several decisions previously made by presidential decree.

The changes will only come into effect at the next election, and so will not apply to Moon personally.

Moon, a former human rights lawyer, has vowed to end what he described as an "imperial presidency" and said in a statement Monday: "I gain nothing from the constitutional change, which gives some of the presidential power to the people, the regional governments and the parliament."

Under the changes, the president will no longer be able to name the chief justice of the constitutional court, with the judges instead choosing among themselves.

Presidential pardons will have to be reviewed by a special committee, and the powerful Board of Audit and Inspection - an internal state inspection agency currently overseen by the president's office - will be given its independence.