South Korea's main opposition Grand National Party (GNP) has won a sweeping victory in the National Assembly by-elections while the ruling Uri Party failed to claim a single seat, south Korean Yonhap news agency reported Sunday.

In Saturday's by-elections, the main opposition party took five out of the six seats up for grabs, increasing its number of Assembly seats to 125 and successfully blocking the ruling party's majority in the 299-member parliament, Yonhap said.

The results represent a significant change in public sentiment toward the governing party, Yonhap quoted political analysts as saying, as five of the six seats were previously Uri Party seats, held by legislators who were forced to step down for electoral law violations.

The by-election's outcome caught the Uri Party completely off guard as the party had been banking on regaining a majority in the National Assembly, it said. The by-elections further diminished the influence of the ruling party's 146 seats in the unicameral house as the National Assembly was restored to its maximum 299 seats.

In the by-elections for local government positions also held on Saturday, of the seven mayoral and gubernatorial posts for contention, the main opposition party's candidates won five, including one for mayor of Hwaseong in Gyeonggi Province, while the ruling party again failed to win any, Yonhap said.