SEOUL, South Korea — Tens of thousands of North Koreans shouted slogans to denounce international sanctions at a rally in central Pyongyang on Thursday, as the communist country vowed to enlarge its atomic arsenal and warned of a "fire shower of nuclear retaliation" in the event of a U.S. attack.

The rally marked the 1950 outbreak of the Korean War, which about 5,000 people — mostly American and South Korean veterans and war widows — also commemorated at a ceremony in Seoul.

The anniversary came a day after President Barack Obama extended U.S. economic sanctions against North Korea, saying its arsenal and the risk of proliferation "continue to pose an unusual and extraordinary threat" to the United States, according to the White House Web site.

The U.S. measures are on top of U.N. sanctions imposed on the North over its nuclear test in May. The U.N. sanctions bar member states from buying weapons from or selling them to North Korea. They also ban the sale of luxury goods to the isolated country and financial transactions.

In Pyongyang, an estimated 100,000 packed the main square, shouting "Let's smash!" in unison while punching clenched fists in the air, footage from APTN in North Korea showed. A placard showed hands crushing a missile with "U.S." written on it.

The isolated, totalitarian regime often organizes such massive rallies at times of tension with the outside world.

North Korea's "armed forces will deal an annihilating blow that is unpredictable and unavoidable, to any 'sanctions' or provocations by the US," Pak Pyong Jong, first vice chairman of the Pyongyang City People's Committee, told the crowd.

State-run newspapers ran lengthy editorials accusing the U.S. of invading the country in 1950 and of looking for an opportunity to attack again. The editorials said those actions justified North Korea's development of atomic bombs to defend itself.

The North "will never give up its nuclear deterrent ... and will further strengthen it" as long as Washington remains hostile, Pyongyang's main Rodong Sinmun newspaper said.

At the rally in Seoul, Minister of Patriots and Veterans Affairs Kim Yang called for North Korea to "abandon all programs related to nuclear weapons and ballistic missiles."

The new U.N. resolution — passed to punish Pyongyang after its May 25 nuclear test — seeks to clamp down on North Korea's trading of banned arms and weapons-related material by requiring U.N. member states to request inspections of ships carrying suspicious cargo.

North Korea has said it would consider any interception of its ships a declaration of war.