The crowds marched off the square, shouting "Let's smash the rats to death!"
Tens of thousands of North Koreans rallied Friday, screaming hatred for South Korean President Lee Myung-Bak and calling for his death over alleged insults during the North's anniversary celebrations.
Troops and civilians packed Pyongyang's Kim Il-Sung Square, pledging to "wipe out the Lee Myung-Bak group, sworn enemy, to the last man on this land and under this sky", the state news agency said.
Crowds demonstrated "resentment and hatred for the group of rat-like Lee Myung-Bak, which hurt again the dignity of the (North's) supreme leadership".
The rally, shown live on state TV, came one day after the North demanded South Korea apologise for the perceived insults or face a "sacred war".
Speakers were shown screaming insults against Lee as the crowd responded with shouts of "Tear the group of rats to death!"
A soldier told the crowd that troops "are determined to punish the rats mercilessly with guns and rifles.
"Let's completely wipe out the (presidential) Blue House and the origin of provocations," he said.
A giant caricature showed the South Korean leader with his throat cut while another depicted him being smashed with a fist.
Crowds marched off the square, shouting "Let's smash the rats to death!" and "Let's safeguard our leader Kim Jong-Un with our lives!"
A TV anchorwoman repeatedly urged Jong-Un to "give us an order now. We are ready to fight, wipe out the group of traitors and achieve unification."
The North in recent months has mounted a vitriolic campaign against Lee. One analyst said Friday's rhetoric was the most extreme for decades against a South Korean leader.
The North charges Lee with "high treason" for restricting mourning visits by South Koreans to Pyongyang after the death of the North's leader Kim Jong-Il last December.
Jong-Il was succeeded by his young son Jong-Un, who has maintained a hostile tone against the South's government.
This week the North accused Lee of "desecrating" mass celebrations marking the 100th anniversary on April 15 of the birth of founding president Kim Il-Sung.
It bridled at anti-Pyongyang demonstrations staged in Seoul and at comments by Lee and conservative media. These questioned the cost of the anniversary celebrations for a nation suffering acute food shortages.
Lee said the estimated $850 million cost of a rocket launch intended to mark the anniversary could have bought 2.5 million tonnes of corn.
"The language appears to be more extreme than against any South Korean president since Park Chung-Hee," said Yang Moo-Jin, of Seoul's University of North Korean Studies, referring to the authoritarian leader assassinated in 1979.
"Since confrontation with the US or the UN is a considerable burden, North Korea appears to be concentrating its attack on South Korea," Yang told AFP.
"It is also apparently designed to extract more public support and solidify unity around Jong-Un. If tension escalates, there could be an armed provocation or limited cross-border conflict."
The rocket, which Pyongyang said was intended to put a satellite into orbit, disintegrated some two minutes after blast-off. The UN Security Council strongly condemned the operation and said it would tighten existing sanctions.
Undeterred, the North vowed late Thursday to launch satellites "one after another" and rejected condemnation of the April 13 launch, seen overseas as a disguised ballistic missile test.
The North said it has already completed an investigation into the cause of the failure. The experience gained would be "a reliable guarantee for greater success in the days ahead", it said.
The North insists its satellite launch was not banned under UN resolutions and did not breach a deal with the United States announced in February.
Under that agreement, the North agreed to suspend a uranium enrichment programme and impose a moratorium on nuclear and long-range missile tests, in return for 240,000 tonnes of food aid.
Washington has suspended delivery of the food and Pyongyang has threatened unspecified retaliation for what it called a hostile US stance. Several analysts believe it will conduct a third nuclear weapons test.