The progressive Uri Party, which supports Roh's reformist progamme, won between 155 and 182 seats in the 299-seat chamber in elections held on Thursday, according to exit polls conducted by television networks.
The conservative Grand National Party (GNP), which controlled the outgoing National Assembly with 137 seats, won between 92 and 114 seats, the exit polls conducted by the country's three major networks said.
The outcome was a dramatic victory for the Uri Party, which was formed only five months ago and held just 49 seats in the outgoing chamber.
But it was less of a rout conservatives in the GNP had feared from an electorate angry at the impeachment the president last month.
The result was also an abrupt shift to the left for South Korean politics, marking the first time in over four decades that the country's parliament has a majority of reformist legislators.
"If exits polls are true, people have rescued our democracy through these elections. They have saved the president. The elections confirm the people are the owners of this country," said Uri party leader Chung Dong-Young.
The 12 March impeachment of Roh was the single key issue in the contest.
The Uri Party asked the public to punish the GNP for backing the vote which was opposed by seven out of 10 South Koreans.
"If exits polls are true, people have rescued our democracy through these elections"
leader, Uri party
"I appeal to the people to help restore the president to power," Chung said after voting in his constituency in southern Seoul.
The Constitutional Court has 180 days to decide whether to reject or endorse impeachment, which would enforce a new presidential election within 60 days.
Turnout in the polls was higher than in the previous election four years ago, according to the National Election Commission. This was a key asset for the Uri Party.
Nearly half of the country's 35 million eligible voters are in their 20s and 30s and are the natural constituency of Roh and the liberal Uri Party, which advocates reform and change and a more independent relationship with the United States.
Some 1,170 candidates competed for 243 directly contested seats, with 56 additional seats to be distributed according to the number of popular votes each party gains.