She is accused of referring to Zimbabwe President Robert Mugabe a "selfish" and "sick" man. The head of state has been elected to power in every vote since 1980 and will soon seek reelection.
The Twitter post was complemented by an illustration of the Mugabe with a catheter.
The New Jersey native is also charged with undermining the authority of or insulting the president.
"Breaking News: High drama in Zim as police currently preferring a charge of attempting to overthrow Mugabe govt against @Martha_ODonovan," Zimbabwe Lawyers for Human Rights tweeted.
On Saturday, O'Donovan's representative failed to have the “subversion” charge thrown out.
"Our client is vehemently denying both charges," her lawyer Obey Shava told AFP, adding that O'Donovan would apply for bail on Monday at the High Court.
On Friday, a human rights lawyer said the arrest was linked to a retweet which did not mention Mugabe by name but referred to a "goblin whose wife and step-son bought a Rolls-Royce."
Mugabe's stepson with his wife Grace is believed to have recently imported two Rolls-Royce, according to local media reports.
Twenty-five-year-old O'Donovan, who works for a TV station in the Harare area, calls the allegations "baseless and malicious."
"Most Zimbabweans do vent on social media about the state of the country, so if they arrest her for this they might as well arrest all Zimbabweans on social media," Munya Bloggo, who created the hashtag #FreeMartha, told BuzzFeed News.
"I don't think anyone is exempt."
The arrest follows Mugabe's appointment of a cybersecurity minister last month.
"This arrest marks the start of a sinister new chapter in the Zimbabwean government's clampdown on freedom of speech, and the new battleground is social media," Amnesty International's deputy regional director Muleya Mwananyanda said in a statement.
Zimbabwe authorities say they tracked the Oct. 11 tweet to O'Donovan's IP address, before taking her into custody on Friday morning.
Tweets continue to be posted from the account, though O'Donovan remains in custody.
O'Donovan is employed with the Magamba Network NGO, which aims to use "creative forms of youth activism to build a democratic & just Zimbabwe," and receives its funds from private foundations.
“It’s one more violation of our already shrunken human rights space,” Magamba co-founder Tongai Makawa told the Los Angeles Times.
“It’s terrible and unfortunate. We produce satirical content that speak the truth to power. I think the nature of the work that we do at Magamba TV would be like low hanging fruit."
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