The leader of the main opposition party offered an apology Wednesday for the departure of former co-chairman Ahn Cheol-soo as he vowed to reform the embattled party.
  

"I feel sorry as the leader of the main opposition party," Moon Jae-in said during a meeting of senior party members.
  

The comments came three days after Ahn, who co-led the main opposition New Politics Alliance for Democracy last year, announced his departure following internal strife with Moon.
  

Ahn had called for the resignation of Moon and party reform following the NPAD's crushing defeat in the parliamentary by-elections in April. He left the party when negotiations with Moon over a leadership change fell through.
  

Moon also pledged to end the "factional politics" by establishing a transparent and fair nomination system ahead of the general elections slated for next April.
 

 The opposition party is seeking to introduce a public panel for each constituency to select candidates.
  

Moon renewed calls to win the next election through the nomination proposal drawn up by the NPAD's reform committee to overhaul the party that has struggled with falling popularity.
  

Ahn, who submitted the letter of defection to the party on Monday, will officially leave later this week when the paperwork is done, according to Ahn's close confidant.
  

Ahn has not made clear whether he has specific plans to create a new party along with other lawmakers or run as an independent.
  

The high-profile defection may result in a stream of similar desertions from the NPAD, with some assuming up to 20 NPAD lawmakers may follow suit by the end of this year.
  

Ahn, the founder of the nation's largest anti-virus software firm Ahnlab Co., co-founded the NPAD with former Chairman Kim Han-gil and Moon, after merging the main opposition Democratic Party and his supporters. (Yonhap)