The FSB and Belarusian security services arrested two men in Moscow.
One of the men also held US citizenship.
Preparations for an armed uprising were in process, the head of the Belarusian KGB intelligence agency Ivan Tertel said in a video.
He said the leadership would have been overthrown and the president, his family and senior officials would have been killed.
Key Belarusian buildings would have been occupied under the plan, Tertel said.
Lukashenko has repeatedly caused fear among the populace, suggesting his opponents are planning a violent coup.
Mass protests continue
As mass protests against him continue, he has also said he had prevented a revolution from taking place like the one in neighboring Ukraine.
After last year's presidential election on August 9, which was widely regarded as manipulated, hundreds of thousands of people had at times called for Lukashenko's resignation and new elections.
While he claimed victory, with 80.1% of the vote, the EU does not recognize Lukashenko as president and has imposed sanctions on Minsk. He relies on Russia as his most important ally.