Machado has acknowledged that she wrote the e-mails, but said she was referring to Maduro's political — not physical — annihilation. She called the accusations "grotesque" and "a mockery" in comments published Friday by Venezuela's El Nacional.
"In Venezuela, proof isn't necessary for someone to be charged, nor is a crime necessary for someone to be jailed," she said.
Prosecutors have cited Venezuelan laws that lay out prison terms of 8 to 16 years for "anyone who, inside or outside the national territory, conspires to destroy the republican political character of the nation."
In a televised interview Friday morning, a communications ministry official, Tania Diaz, said she also possessed intercepted phone calls in which Machado allegedly exhorted protesters to violence, and accused her of trying to foment "civil war."
Machado has not been taken into custody. But if jailed, she would be the second major opposition figure to be locked away on charges of trying to overthrow the Maduro government during the street demonstrations that flared then fizzled out earlier this year. At least 43 were killed, including protesters and government security forces, during three months of street clashes.
Leopoldo Lopez, a co-founder with Machado of the "Popular Will" party, has been held in a military prison since February, drawing criticism from the United Nations Committee Against Torture.
"We are deeply concerned by what appears to be the Venezuelan government's continuing effort to intimidate its political opponents through abuse of the legal process," U.S. State Department spokesperson Marie Harf said in a statement following Machado's indictment.
"The charges against Machado raise concerns once again about Venezuela's arbitrary use of prosecutorial powers to silence and punish government critics," Harf said.
Though she is one of the most visible leaders of Venezuela's opposition, Machado has less of a political following than Lopez or governor Henrique Capriles, who narrowly lost to Maduro in the April 2013 presidential election.
Machado was stripped of her national assembly seat by Maduro's ruling party this year when she testified at a special session of the Organization of American States denouncing the Maduro government for right abuses.
In a speech this week, Maduro called her "a murderer."
The embattled president — the hand-picked successor of the late Hugo Chavez — faces an increasingly grim economic outlook.
The price of Venezuelan crude oil — the source of nearly all government revenue — fell below $62 Friday, its lowest level in five years. The unofficial exchange rate for Venezuelan bolivars jumped to 170 per dollar, up from 95 at the start of last month, and Venezuelans are facing shortages of food, medicines and other basics.
A survey by the leading polling firm Datanalisis said Maduro's approval rating slipped in November to 24.5 percent. More than 85 percent of those surveyed said the country was headed in the wrong direction.
Maduro's government has yet to set a date for next year's parliamentary elections.