SEOUL, June 5 (Xinhua) -- Nearly half of the legislators' seats were left empty in the first session of South Korea's new parliament Thursday, as opposition parties boycotted the session in protest at the controversial trade pact to resume U.S. beef imports.

    Only grim-faced ruling party lawmakers attended the opening session, a day after the conservative party suffered a crushing defeat in local by-elections following growing public discontent toward the impending resumption of beef imports. Despite their attendance, the new parliament did not officially convene.

    Three opposition parties, led by the United Democratic Party, agreed Wednesday to indefinitely boycott the new National Assembly, demanding the government completely renegotiate an April 18 beef accord with Washington.

    Under the April trade pact, South Korea agreed to almost entirely remove its restrictions on U.S. beef imports, including those on the age of butchered cattle. The deal triggered immense public concern over mad cow disease, the main cause of Seoul's suspension of imports in 2003.

    Tens of thousands of citizens have taken to the streets almost every day since last month, accusing the government of putting their health at risk for the sake of better economic and political ties with Washington.

    Claiming it would be a "betrayal" to the public to convene the parliament now, opposition legislators have instead been joining the nationwide street protests, using the beef deal as political leverage against the Lee Myung-bak government.

    The new legislature was required by law to convene seven days after its four-year term began, which fell on Thursday. Moreover, discussions on new parliamentary committees -- which must be established by Saturday according to the law -- have also been pushed into the background amid the escalating partisan battle over U.S. beef imports.

    South Korea's parliament has failed to convene in accordance with law on only one other occasion in history. In 1996, the opposition party boycotted the opening session for over a month amid an uproar over the ruling party's alleged irregularities in the general elections.

    In a bid to soothe the mounting public criticism, the South Korean government has been moving to encourage private companies from both countries to agree to voluntarily restrict trading beef from cattle older than 30 months.

    Under the initial pact, Seoul agreed to first open its market to beef from younger U.S. cattle, with the 30-month limit to eventually be removed if the U.S. implements tougher animal feed regulations.

    Three opposition parties last week filed a constitutional appeal to block the resumption of imports, and have agreed to a joint campaign for the collective resignation of the Cabinet members under the Lee administration. Their earlier motion to oust Agriculture Minister Chung Woon-chun was narrowly dismissed in a parliamentary vote last month.