In the ceremony on Thursday, North Korea set off explosives in three of its four tunnels at the Punggye-ri site, according to dispatches by reporters at the scene. The fourth tunnel had already been closed for fear of contamination after the North’s first nuclear test in 2006.

The North Koreans also blew up test-observation facilities, as well as barracks for site personnel and a metal foundry, the reports said.

Two dozen international journalists were invited to witness explosives rigged inside the tunnels, and they were then escorted outside to viewing decks 500 yards away, where they filmed the detonations.

North Korea invited mostly TV journalists to ensure that its action would be broadcast worldwide.

Even before Mr. Trump canceled the summit meeting, some analysts feared the moves would be reversible — as when the North disabled and then restarted a nuclear reactor years ago. But Thursday’s reported demolition at the test site was the first concrete step North Korea had taken under Mr. Kim toward what Washington had been hoping would be a complete nuclear dismantlement.

North Korea announced last month that it would end all nuclear and long-range ballistic missile tests, as well as close the test site. It said it no longer needed to conduct such tests because the country already had nuclear weapons and wanted to focus on rebuilding its economy.

It is still not known whether the country will give up its nuclear arsenal — an uncertainty that has only intensified now that the much-anticipated meeting between Mr. Trump and Mr. Kim has been canceled.

Since no weapons experts were present Thursday, it is not even clear whether North Korea destroyed all of the tunnels at the test site, or if the explosions would prevent the site from being used again.