South Korea's political opposition leaders today strongly objected to a plan to elect the late president Park Chung Hee's successor under the present constitution.

Kim Young Sam, president of the New Democratic Party, said his party "cannot agree" to the plan unveiled earlier today by Acting President Choi Kyu Hah to elect Park's successor soon under existing rules and sometime later hold another election under a revised constitution.

Kim reiterated his position that the constitution should be revised immediately to provide for direct election by popular vote of the next president.

Nevertheless, sources in Seoul said it is likely that Choi will proceed with the election of a caretaker president and that he is the man most likely to succeed Parks, who was killed on Oct. 26.

The date for the election has not been announced, but sources said it would probably be sometime in December, although it legally could be delayed until Jan. 25.

Following Choi's plan would mean that whoever is chosen is almost certain to be a person acceptable to the followers of the late president, who ran the country with a strong hand and tolerated little dissent throughout most of his 18-year rule.

The choice would be made by the National Council for Unification, a group of 2,562 delegates elected last year under Park's constitution who promptly chose him for a six-year term.

The council, by law, does not include members of any political party, reflecting Park's view that no politician should play a role in choosing a president, and most won membership on the council became of proven loyalty to Park.

A candidate for the council's consideration must have endorsements from at least 200 council members. It would be almost impossible for any opposition figure to obtain that many endorsements.

In a nationally televised statement this morning, Choi declared that the old constitution should be followed for the time being to preserve stability in a time of "national emergency," and he warned that political divisiveness might encourage an attack by the North Korean Communists.

He said the successor should not serve out the full five years of Park's term. "As soon as realistically possible," he said, the constitution should be amended and a new election held under those new rules.

But sources in Seoul noted today that Choi was vague on many questions. He did not spell out how long the interim president should serve and he did not say that the amendments should provide for a direct popular election, as opponents have demanded.

The pro government Democratic Republican Party, without expressly approving or disapproving of the plan, announced that it would not attempt to field a candidate in the election.

That would seem to leave the field virtually open to Choi, the career bureaucrat and former foreign minister who had been one of Park's closest friends and advisers. He had been given the largely ceremonial job of prime minister in 1976 and managed to stay close to Park because he lacked any separate political base that might have made him a threat to the late president.

In a statement issued a few hours after Choi addressed the nation, opposition leader Kim rejected his plan and insisted on a quick constitutional amendment to provide for a popular vote.

"This is the only way to stabilize the nation, to deter North Korean provocations, and to attain the support of the world," he said.

"Choi's announcement has given the people a great disappointment," he said, stressing he thought the acting president should have conferred with opposition politicians before making it.