North Korea is holding what is expected to be one of its biggest celebrations ever, marking the 70th anniversary of the ruling Workers' Party.
A cavalcade of armoured vehicles and ballistic missiles rumbled through the capital Pyongyang, accompanied by marching troops.
North Korea's supreme leader Kim Jong-un presided over the ceremony, but no world leaders attended.
He said that North Korea was ready to fight any war waged by the US.
"The party's revolutionary armament means we are ready to fight any kind of war waged by the US imperialists," Mr Kim said at Pyongyang's main Kim Il-sung Square.
The celebrations featured thousands of soldiers - many carrying red party flags or banners - in tight square formations goose-stepping to martial music.
Tanks rolled past the podium where Mr Kim spoke, with an aircraft flypast forming the number 70.
The parade is the North Korean leader's message as much as his words - it is meant to send a signal that there is military force here, and that it is a united military force, says the BBC's Steve Evans in Pyongyang.
When it announced plans for the anniversary earlier this year, the government spoke of "cutting-edge" weaponry suitable for modern warfare, and the day's event will be closely watched for any new military hardware indicating the North's military development.
China, North Korea's closest ally, sent a senior Communist Party official to the anniversary - a signal of the closeness between Beijing and Pyongyang, our correspondent says.
The North Korean leader began the day by paying respects to his late father and grandfather at the Kumsusan Palace of the Sun in Pyongyang, official media reported.
Officials did not divulge details of the day's celebrations, although thousands of Pyongyang citizens have been seen in public squares across the city practising for a torchlight parade to be held in the evening.
A stage has also been set up on a river for a late-night concert featuring the all-female Moranbong Band - North Korea's most popular musical group.
North Korea's nuclear programme has long been a cause for alarm in the region.
Pyongyang is believed to have conducted three underground nuclear tests and is threatening a fourth, despite international condemnation and sanctions.
Last month, it announced it was restarting its main nuclear facility Yongbyon and that it was improving its nuclear weapons "in quality and quantity".
The North claims it has made a nuclear device small enough to fit into the warhead of a missile.
US officials, however, have cast doubt on this claim and experts say it is difficult to assess the progress North Korea has made on miniaturisation.
North and South Korea remain technically at war because the 1950-1953 war ended in an armistice, not a peace treaty.