Appearing before thousands of delegates, the North Korean leader said: “Unprecedented results have been accomplished.”
This year, North Korea announced its fourth nuclear weapon test and that it sent a rocket into space.
The showpiece congress, the first in 36 years, is a chance for Kim Jong-un to cement his power.
More than 100 foreign journalists were invited but were barred from the April 25 House of Culture, where the party congress is being held.
They were instead taken on a tour round a wire-making factory.
News of Kim Jong-un’s comments came via state television, which showed him on stage speaking to a packed venue.
This is the seventh meeting of the North Korea’s Worker’s Party and it is being closely scrutinized for any signs of political or economic change.
Few details were released before the congress, but it is due to elect a new central committee, which appoints a Politburo – the central decision-making body of the Communist party.
The appointments will be watched carefully.
In 2013, Kim Jong-un had his uncle executed for “acts of treachery” and there have followed many reports of purges of high-profile figures.
Some experts have said that Kim Jong-un’s sister Kim Yo-jong is tipped for promotion.
Pyongyang was spruced up ahead of the event, with North Koreans planting flowers and hanging up huge banners with slogans like “Defend the Headquarters of the Korean Revolution at the Cost of our Lives”.
No congress was held during the rule of Kim Jong-un’s father, Kim Jong-il. His death in 2011 brought Kim Jong-un to power when he was still in his 20s.
The 1980 congress, held before Kim Jong-un was born, saw Kim Jong-il presented as successor to the North’s founding leader Kim Il-sung.
Despite his death in 1994, Kim Il-sung, who has been named North Korea’s “eternal president”, still officially presides over the latest congress, which is expected to run for several days.
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