HARARE (Reuters) - A U.S. citizen accused of attempting to subvert President Robert Mugabe s government was released from Zimbabwe s maximum security jail on Friday, a day after the High Court ordered her to be freed on bail, Reuters witnesses said. Judge Clement Phiri ruled on Thursday that there was a  patent absence of facts  in the state s case against 25-year-old Martha O Donovan, whose arrest last week centered on accusations she insulted 93-year-old Mugabe in a Twitter post. She denies the accusation. Reuters witnesses saw O Donovan leaving prison in a United States embassy vehicle. O Donovan and her lawyers did not speak to reporters waiting outside Chikurubi Maximum Prison on the outskirts of Harare. State prosecutors accuse O Donovan of writing a Twitter post in October calling Mugabe a  selfish and sick man .  The government has since last year been targeting activists and government critics who use social media to speak out against Mugabe, cash shortages in banks and a foreign currency crunch that has caused a sharp rise in prices. Amnesty International said it feared O Donovan would not be the last to be arrested in the government s  clamp down on social media platforms .  Activist pastor Evan Mawarire, whose #ThisFlag movement last year organized the biggest stay-at-home demonstration in a decade, is on trial on a charge of subversion. He faces a separate trial for a similar offence. 