The allegations are the latest in a torrent from the government, which says three months of protests this year were a veneer for a US-backed conspiracy to unseat the successor to late socialist leader Hugo Chavez.
Opposition leaders deny and deride those accusations as a smokescreen for a struggling government, saying the demonstrations, in which 42 people died, were a spontaneous response to economic hardship, insecurity and state repression.
Maduro's top officials lined up for a much-touted news conference, which all local broadcasters were required to carry live, to denounce Machado and other opposition figures for "assassination" and "coup" plots against the president.
They also accused US envoy to Colombia, Kevin Whitaker, of being involved and in touch with Machado. A spokeswoman for the US Embassy in Colombia did not immediately respond to requests for comment. "What we are presenting is part of a criminal investigation," ruling Socialist Party official Jorge Rodriguez said, showing emails he said were proof of the plans.
Machado, 46, who was instrumental in calling Venezuelans onto the streets for three months of demonstrations against Maduro this year, scoffed at the accusations as "infamy" via Twitter.
Rodriguez, a psychiatrist as well as a politician, showed iPad screenshots of various emails he said were from Machado, showing language typical of "serial killers".
"We need to clean up this rubbish, starting at the top, taking advantage of the global climate with Ukraine and now Thailand, as soon as possible," read one. "The time has come to join forces, make the necessary calls and obtain the funds to annihilate Maduro."
Machado and another hardline opposition leader, Leopoldo Lopez, were the main proponents of the protests that began in February. Lopez was jailed in mid-February and his wife, Lilian Tintori, who is also close to Machado, met Pope Francis at the Vatican on Wednesday during an international tour to garner support for her husband. Three generals and an army captain were also imprisoned in March on charges of "coup" plots.