The new leader of the main opposition party on Wednesday urged President Park Geun-hye to offer an apology over the guilty verdict handed down to a former spy chief for intervening in the 2012 presidential election.

The demand by Moon Jae-in came two days after the Seoul High Court sentenced Won Sei-hoon, who headed the National Intelligence Service (NIS) from 2009-2013, to three years in prison on charges of violating election laws.

The appeals court said Won "willfully neglected" an online smear campaign carried out by his subordinates against the rival candidates of Park, the then ruling party hopeful.

Park won the election, edging out Moon by a slight margin of 3.6 percentage points. Moon is back in the political spotlight after becoming chairman of the New Politics Alliance for Democracy.

"President Park should offer an apology on the issue, though it occurred during the Lee Myung-bak administration," Moon said, referring to Park's predecessor.

It was not immediately clear whether Park will make any comment on the court ruling.

In 2013, Park said she did not receive any help from the NIS in the 2012 presidential election.

Moon also called on Park to reform the NIS to ensure that the spy agency will not meddle in politics.

In South Korea, NIS agents are legally mandated to remain politically neutral. (Yonhap)