The government said it would seek Wuthipong Kochathamakun's extradition from neighboring Laos and pledged to stop anything which could cause unrest in Thailand, where the opposition has been largely silenced since a 2014 coup.
"They're trying to force terrorism charges on me," Wuthipong said in an interview posted on YouTube. "This is a very poorly done setup."
The police drew a link from their discovery not only to the red shirts supporters of ousted populist leader Thaksin Shinawatra, but also to an influential Buddhist temple that is at odds with authorities.
They did not give details of the alleged plot to kill junta leader Prayuth Chan-Ocha but said they had discovered the weapons at a house belonging to Wuthipong, who fled Thailand at the time of the coup.
The government played down any risk to Prayuth's life, but said that it would take action against any threat to stability as it tries to bring about political reconciliation before eventual elections.
"Whatever causes unrest, we will put it out," Defense Minister Prawit Wongsuwan said. Critics accuse the government of using security threats to justify the army's continued grip on power.