President Chun Doo Hwan was assured today of a seven-year term as president of South Korea after a sanitized election from which almost all prominent politicians of the past were banned.

With most of the votes counted today, Chun's supporters were winning about 70 percent of the seats in a new electoral college that will choose the new president in two weeks. A new organization that styles itself as an opposition party got about 7 percent of the seats and independents and minor party candidates got the rest.

The outcome was no surprise. Virtually all former opposition leaders had barred by edicts issued during Chun's military rule last year. The former general's party leaders had set a target weeks ago of winning 75 percent of the seats and were falling slightly below that goal.

Besides heading a government that devised and controlled the election ground rules, Chun also benefited from the warm embeace last week by President Reagan. U.S. support is important politically in this country where 30,000 American troops help discourage a communist attack from North Korea. Reagan promised they would not be withdrawn.

The results assure seven years of governing for the 50-year-old Chun, successor to Park Chung Hee, whose assassination 16 months ago touched off a period of instabililty and martial-law rule. Under the new constitution, Chun cannot succeed himself.

A spokesman for Chun's Democratic Justice Party, Park Kyung Suk, asserted that the vote demonstrated popular support for Chun and said it "also expresses the people's aspiration for stability." After a long, wracking period of upheavals, stability had been the party's main platform plank.

The main opposition party, Deocratic Korea Party, accepted the defeat calmly. "We will have to make a new start to win the coming National Assembly elections," said the party's spokesman, Kim Hyung Nai.

Chun's new party was created by a group of his former mililtary colleagues who left the military to take up roles in private life. It also includes a number of leaders of South Korea's biggest business enterprises and an assortment of former officials.

The opposition party is headed by its presidential candidate, Yoo Chi Song, who had played a minor role in an opposition party that flourished in the final days of Park's regime. But all prominent politicians of the past were barred either on corruption charges or on political grounds as part of a massive purge Chun launched last year after assuming power in a military crackdown.

The most prominent opposition leader, Kim Dae Jung, was sentenced to death for sedition but his sentence was commuted to life imprisonment. Another opposition leader, Kim Young Sam, abandoned politics after spending weeks under house arrest. Chun's military regime purged about 835 political figures, some 260 of who later were permitted back into politics.

Under a new election law enacted during Chun's interim presidency, the electoral college campaign was a tame affair with all activities regulated by the government's central election management committee. Candidates were barred from holding private rallies or driving campaign trucks and could state their positions only at authorized ralliles or in sanctioned television and radio speeches.

The campaign management committee regulated the size of posters and the spending of money. Crowds were slim at the ralliles authorized by the government, although the turnout in yesterday's voting neared 80 percent -- not a high response by South Korean standard.

Indirect election of a new president had been strongly opposed last spring by opposition leaders and by some members of the party that had been headed by president Park. They had called for a direct popular election but Chun's military government later devised the electoral college arrangement. Park had been elected by a similar electoral college which he tightly controlled. Altogether, about 9,300 candidates filed to run for the college's 5,278 seats. a