The Sino-Soviet confrontation over who is the legitimate representative of Cambodia became the first controversy before the 34th U.N. General Assembly at its opening session today.

Ambassador Ha Van Lau of Vietnam demanded the ouster of representative of the Pol Pot government from the General Assembly chamber. The government they represent was overthrown by an invasion of Vietnamese troops last January.

Vietnam's ambassador said a delegation from the Cambodian government of Heng Samrin, which was installed by Vietnam, has come to the United Nationals and he urged that it be recognized by the General Assembly.

Assembly President Salim Ahmed Salim of Tanzania referred the dispute to the nine-member credentials committee and requested a report back to the General Assembly Friday morning.

The United Nations decided earlier this year to continue to recognize Pol Pot even though his government had been driven from the capital, Pnom Penh, and was fighting a guerrilla action from stongholds, principally in western Cambodia. That decision was based largely on the world body's traditional opposition to invasions across national frontiers.

China has been Pol Pot's strongest supporter while the Soviet Union backs Vietnam and Heng Samrin.

When the Cambodian dispute arose at the Havana conference of nonaligned nations earlier this month, the compromise is not available to the United Nations of which Cambodia is a member.

For the United States, the choice is between "equally obnoxious tweedle dum and tweedle dee," one senior official said. The United States supported the Pol Pot delegates when the issue last arose, but it is not clear how it will vote in this General Assembly.

The Assembly's opening session welcomed the Caribbean Island nation St. Lucia as the U.N.'s 152 member. A former British colony, St. Lucia became independent last Feb. 22. It has a population of about 120,000 in an area of 238 square miles.

Salim, 37, who has represented Tanzania here since 1970, succeeded Colombia's Ambassador Indalecio Lievano as president of the General Assembly. In his opening speech, Salim called for self-determination for the Palestinians and denounced Israeli's "senseless bombings of civilian targets."

"The core of the Middle East problem is the continued denial of the inalienable rights of the Palestinian people to self determination, including the right to establish an independent state," he said.