Making the startling disclosure in an interview with the US newspaper San Francisco Chronicle, Jordan's King Abd Allah II said the plot failed after five trucks loaded with 17.5 tonnes of high explosives were seized by the police.
"It was a major, major operation," King Abd Allah said. "It would have decapitated the government."
Casualties would have been "in the thousands," he added. "It could not have been more sinister."
The Jordanian king is visiting California and spoke in San Francisco on Friday. He is scheduled to meet US President George Bush in Washington on Wednesday.
The explosives were apparently intended for an attack on the Jordanian prime minister's office and the intelligence ministry.
But the plot was thwarted after two suspected terrorists were arrested two weeks ago.
King Abd Allah said that, although the trucks came from Syria, he was "completely confident that Syrian President Bashar al- Asad did not know about it."
The king said European anti-terrorism experts were aiding the Jordanian police investigation.