North Korea is holding mass celebrations for the 60th anniversary of the ruling Workers Party.
Supreme leader Kim Jong-il smiled and applauded as thousands of goose-stepping troops filed through the capital, Pyongyang's, main square.
There has been speculation Mr Kim could use the occasion to name his successor.
A senior North Korean official said the Stalinist state would "mercilessly crush" US and Japanese forces if they threatened his country.
This year's celebrations are expected to be the largest for some time, a huge display of confidence after a period of uncertainty both at home and abroad, says the BBC's Seoul correspondent Charles Scanlon.
A vast crowd of well-drilled civilians roared slogans in support of Mr Kim during the festivities.
A senior Chinese envoy was a prominent guest. China has given crucial economic support to its communist neighbour, but has been pressing North Korea to give up its nuclear weapons programme.
Comments by Vice-Marshal Kim Il-chol, broadcast on state television, reflected the tension with the international community that the North's nuclear stance has generated.
Kim has several children
His eldest son disgraced himself in Japan
Therefore, power may pass to a younger boy, Kim Jong-chul
"Our People's Army will continue to firmly safeguard the party and the socialist fatherland with its invincible military power, and should the US and Japanese imperialists and their follower forces persist in lighting the flames of war in this land, we will mercilessly crush the aggressors and achieve the historic cause of the fatherland's reunification," he said.
On Sunday night, the main speech was delivered by the deputy leader, Kim Yong-nam.
He stressed the party's commitment to its "army first" policy, which puts a strong military at the centre of the state's priorities.
As many as one million soldiers and civilians are expected to take part in marches and rallies through the capital's central square.
There has been speculation that Mr Kim could use the occasion to name his successor.
Analysts say he may begin by naming one of his three sons to a senior post in the party or army - but they also say there is no clear indication it will take place during the current festivities.
The North Korean leadership has shown growing confidence in recent weeks, after receiving pledges of substantial aid from overseas in return for giving up its nuclear weapons.
It has revamped the state rationing system and ordered out foreign aid workers in what looks like an attempt to reassert its authority.