Tension on the Korean peninsula spiked after the North declared Saturday it would accelerate its nuclear bomb-making program by producing more plutonium and uranium, two key ingredients.
The North also threatened war with any country that tries to stop its ships on the high seas as part of new Security Council sanctions passed in response to Pyongyang's May 25 nuclear test. It conducted its first test in 2006.
South Korean President Lee Myung-bak departed for summit talks in Washington on Tuesday with President Barack Obama that are expected to be dominated by the North's nuclear and missile programs.
In Pyongyang, a massive crowd of North Koreans packed the capital's main square in a rally to condemn the U.N. resolution, footage from APTN in North Korea showed. The isolated, totalitarian regime often organizes such rallies at times of tension with the outside world.
APTN North Korea estimated the crowd at about 100,000.
"We strongly condemn and wholly reject the U.N. Security Council's resolution on sanctions, fabricated at the instigation of U.S. imperialism hell-bent on its attempt at stifling" the North, Kim Ki Nam, a top Workers' Party official, told the crowd.
Participants clapped and chanted "Condemn! Reject!" in unison, pumping clenched fists into the sky.
The North is believed to have enough weaponized plutonium for at least half a dozen atomic bombs, and a U.S. government official said last week that Pyongyang may be preparing for another nuclear test, its third.
The official spoke on condition of anonymity in order to discuss the unreleased information and provided no details.
Also Monday, Yonhap news agency quoted an unidentified intelligence official as saying the North may have already built two to three underground test sites near its known Punggye-ri site in the remote northeast, where it conducted its first and second tests.
South Korea's Defense Ministry and National Intelligence Service said they could not confirm the reports.
A news report from Moscow quoted an official in the Russian military general staff as saying there has been a decrease in visible activity around North Korea's nuclear facilities in recent days.
This could either indicate that the North has prepared for a new underground nuclear test or is taking a break, according to the state-owned RIA-Novosti news agency. It did not name the official, and the general staff could not immediately be reached for comment.
North Korea has also been preparing to fire an intercontinental ballistic missile capable of striking the United States, U.S. officials have said. The North says the nuclear and missile programs are a deterrent against the United States.
Washington fears that cash-strapped North Korea will sell its nuclear technology to rogue nations, spreading the atomic threat.
The regime has also warned it cannot guarantee the safety of South Korean and U.S. navy ships sailing near the disputed western sea border, raising the specter for a maritime confrontation. The area is the scene of two bloody maritime skirmishes between the Koreas in 1999 and 2002.
South Korea's navy chief of staff said a maritime skirmish could occur "at any time" and that his forces were prepared.
"We will cut off the enemy's wrist even if they touch the tip of our finger," Jung Ok-keun said at a ceremony marking a deadly naval clash with North Korea in 1999.
But the Defense Ministry said Monday it has spotted no unusual moves by the North's military.
The strong ties between South Korea and the United States are a thorn in the side of wartime foe North Korea, which accuses the two countries of plotting an attack to topple the communist regime. The allies deny harboring any such intention.
But President Lee said his country's ties with the United States are "key" at a time of "intensifying" security crisis because of North Korea's nuclear and missile tests.
"I will use this summit to reconfirm the strong Korea-U.S. alliance," Lee said in a radio speech before his departure for Washington.
The two Koreas remain technically at war because their three-year conflict ended in a truce, not a peace treaty, in 1953, and they remain divided by a heavily fortified border. The U.S. has 28,500 troops in South Korea.
The two Koreas signed an accord to ease military tensions and promote economic cooperation nine years ago Monday. However, ties have significantly frayed since Lee, a conservative who advocates a hard-line approach, took office last year. The North responded by cutting off ties and halting joint business projects.
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Associated Press photographer Young-joon Ahn contributed to this report from Imjingak, South Korea.
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