North Korea is making an unprecedented effort to show itself off internationally this weekend as a modern, economically strong country ready to be ruled by a new generation of young leaders.

The Communist Party's first congress in 10 years is a showcase event stressing those features -- youth and economic progress -- that have not been exactly at the forefront of its international image in the past decade.

To much of the world, North Korea has remained for years a mysterious, secrecy-shrouded Communist empire ruled by an aging leadership and dominated by the all-inclusive worship of its 68-year-old president, Kim II Sung.

It has also projected the unpleasant picture of a country that cannot pay its bills. A mid-1970s recession left it embarrassingly in hock to Japanese traders and forced it to scramble in an unseemly fashion for foreign currency to pay for its modernization.

Clearing away such musty perceptions seems to be a major purpose of the North Korean Workers' Party Congress, which opened yesterday in Pyongyang before a large international audience.

Japanese businessmen who deal with that country described this week a massive campaign to polish things up for the 117 foreign delegations and more than 100 foreign media representatives.

Billed in Pyongyang as the "100-day combat," it included a big outlay for quick imports of the latest equipment -- elevator systems, flourescent lights, aluminum door frames, dump trucks, and mining machinery.

The "100-day combat" was timed to end with the opening of the party congress and businessmen here say it was designed to put a bit of gloss on an economic machine that had gotten a bit rusty in the hard-luck times of the late 1970s. Kim, in a 4 1/2-hour speech yesterday, proclaimed the effort a brilliant success and boasted of gains achieved in industrial production in recent years.

Putting a youthful face on the country's leadership seemed the second goal. Although there is no indication that Kim or the others are preparing to step down immediately, the weekend's script was designed to show off a younger leadership in the wings.

The headline event in that scenario was the emergence of Kim's son, Kim Chong II, in the public limelight. Most analysts assume the young Kim has long been designated his father's successor, but he has never appeared in a forum with foreigners and his name was never seen in the party press.

Yesterday, according to accounts in Japanese newspapers, he appeared prominently in the front row of a group including the most important foreign visitors and national leaders. His name appeared fifth on a list of members of a steering committee and the only ones ahead of his were those of his father and three other aging leaders. When the names were read over a public address system, young Kim's was pronounced with emphasis and a murmur of surprise spread through the audience, the Japanese accounts reported.

The scenario clearly placed young Kim at the top of the heap of potential leaders, although no succession announcement was forthcoming.

The twin themes of youth and modernization seem directed most of all at the many delegates from Third World nations, particularly in Africa. President Kim is known to consider himself a successor to the late president Tito of Yugoslavia as a father figure for the nonaligned world. Although prominent guests came from the Soviet Union and China, Third World figures got the most attention. Chief among them were Robert Mugabe of Zimbabwe and Guinea's Ahmed Sekou Toure.

Mugabe was greeted with a big hug from President Kim on arrival at the Pyongyang airport and treated to a special banquet on the eve of the congress, while Sekou Toure was seated next to the president's son on the platform yesterday.

The president's lengthy speech did not contain any startling departures or new overtures.

Some analysts had expected a major new initiative for getting reunification talks started again with South Korea. The president spelled out a 10-point program that was essentially a rehash of many old approaches.

But it was coupled with a strong attack on the new Seoul government, which is headed by former Gen. Chun Doo Hwan. One passage of the speech seemed to hint that there is no possibility of getting negotiations started until the Chun government is swept out of power.

Once-promising working-level talks designed to pave the way for eventual unification have been stalled for several weeks and the North's propaganda barrages against the South have resumed with intensity. Most analysts believe there is no longer a possibility of resuming the talks, which were supposed to prepare the way for a meeting of prime ministers.

The president's criticism of the United States was moderate, although he attacked American support for the Chun government. Kim also revived his offer to meet with the United States directly to replace the 1953 armistice agreement with a full peace treaty. But he did not mention the possibility of South Korea joining those talks, and the United States has refused the offer of any discussions unless the South takes part.