The destruction of the highly visible symbol of North Korea's long-secret nuclear program came just a day after the country released details of its program.
"They fired a warning flare and then in three minutes the whole thing came tumbling down in a massive cloud of smoke," Amanpour said.
"There was a moment of stunned silence as the magnitude of what had happened sunk in," Amanpour said.
U.S. State Department officials and observers from the International Atomic Energy Agency (IAEA) watched from a reviewing stand on a ridge about 1,000 yards away, she said.
"This is a very significant disablement step," the U.S. envoy to North Korea, Sung Kim, said.
Nuclear experts say that the plant's destroyed central water-cooling tower would take a year or longer to rebuild if North Korea were to try using the plant again.
"This is a critical piece of equipment for the nuclear reactor," said analyst John Wolfsthal, of the Center for Strategic and International Studies, who has been following North Korea since the 1980s. "Without this facility, the reactor can't operate and can't produce more plutonium for weapons."
North Korea has been dismantling other parts of the facility under the watchful eyes of representatives of the five other nations, including the U.S., that have been involved in six-party talks aimed at ending Pyongyang's nuclear weapons program.
On Thursday, North Korean officials turned over to China a 60-page declaration, written in English, that details several rounds of plutonium production at the Yongbyon plant dating back to 1986.
In it, North Korea acknowledges producing roughly 40 kilograms of enriched plutonium -- enough for about seven nuclear bombs, according to the U.S. State Department.
In response, Bush said he would lift some U.S. sanctions against North Korea and remove the country from the State Department's list of state sponsors of terrorism.
But he made clear that other sanctions remained in place on North Korea -- which has been on the terrorism list since its alleged involvement in the 1987 bombing of a South Korean airliner which killed 115 people.
"The United States has no illusions about the regime in Pyongyang," Bush said. "We remain deeply concerned about North Korea's human rights abuses, uranium enrichment activities, nuclear testing and proliferation, ballistic missile programs and the threat it continues to pose to South Korea and its neighbors."
U.S. analysts will pore over the document to resolve Washington's outstanding concerns, which include questions about the extent of North Korea's proliferation of nuclear technology and the status of any uranium enrichment program.