North Korea is holding its most important political event in decades where leader Kim Jong-un will cement his status and set out his vision.
Thousands of delegates are meeting for a choreographed show of support being seen as an unofficial coronation.
The BBC's Stephen Evans in Pyongyang says Kim Jong-un is inside the hall, with guards lined up outside the venue.
Its run-up saw attempts at missile launches and speculation that the North will conduct a fifth nuclear test.
It is the seventh congress of North Korea's Worker's Party and the first since 1980. It is being held inside the April 25 House of Culture, now covered in vast red and gold banners and massive images of the current leader's father and grandfather.
In pictures: Inside Pyongyang as congress begins
But the event remains shrouded in secrecy. Foreign journalists invited to cover the congress have not yet been allowed inside the venue. State television is not currently broadcasting the event.
In the Pyongyang press pack - Stephen Evans, BBC News
Control doesn't even begin to describe the situation.
In our team of four, each person has a minder. They are people in dark suits and they step in very quickly if they don't like what they see or hear.
They come straight into our hotel room unannounced, and in one case we were told to erase footage we had shot. I was even followed into the urinal.
I think they get a lot of pressure. I think there are people watching and listening to our output and saying to the minders every evening: How on earth did that happen?
National and Workers' Party flags are lining the streets along with banners that read "Great comrades Kim Il-sung and Kim Jong-il will always be with us" and "Defend the headquarters of the Korean revolution at the cost of the our lives".
No congress was held during the rule of Kim Jong-il - Kim Jong-un's father. His death in 2011 brought Kim Jong-un to power when he was still in his twenties.
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The agenda and duration of the event is not known but experts say Kim Jong-un is likely to declare his so-called "byongjin" policy, which is the simultaneous push towards economic development and nuclear capability.
It could also see a new generation of leaders put in place.
The meeting will elect a new central committee, which appoints a Politburo and other key political roles, and many say loyalists to the current leader will be rewarded with high profile posts.
Who he chooses 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 in the establishment.
Some experts have said that Mr Kim's sister Kim Yo-jong, with whom he attended school in Switzerland, is tipped for promotion.
Many observers will scrutinise announcements carefully to evaluate the North's commitment to a planned economy and hints at economic reform, but the congress is also being seen as the public stage for Kim Jong-un to define his leadership for the years to come.
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