Digital rights advocacy group Paradigm Initiative has criticised Zimbabwe's government over the purported use of insult laws to justify the arrest of American journalist Martha O'Donovan.

O'Donovan was arrested for a post reportedly directed at President Robert Mugabe. She was later released on bail after spending at least three days in a maximum security facility, amidst several calls for her release including a Twitter support with the hashtag #FreeMartha.

She is alleged to have retweeted a post originally sent via the Twitter handle @matigary in which the president is called a "sick and selfish man", on 11 October.

O'Donovan's retweet was considered to be 'undermining authority of or insulting the President.' Authorities also believe the journalist is the author of the tweet based on a trace to her IP address.

However, the 25-year old American denied the claim as "baseless and malicious." Her denial was strengthened by reports that the Twitter handle has been active since she was jailed on Friday 3 November.

According to the Zimbabwe Lawyers for Human Rights, which offered legal assistance to O'Donovan, police officers obtained a search warrant to confiscate her electronic devices.

The retweet earned O'Donovan a charge of subversion and attempting to overthrow the Mugabe government. It carries a sentence of up to 20 years in prison.

She already made a court appearance on Saturday 4 November before a Zimbabwe High Court judge ruled in her absence to grant US$1,000 bail on the grounds that the state did not provide alleged tweets which reportedly called on people to protest against the government.

Paradigm Initiative states: "For starters, the tweet forming the basis of arrest does not refer to the President. Further, even if it referred to the President, citizens and all residents are guaranteed the freedom to criticize public officials including presidents. In this regard, insult laws such as the one purported to be used in this matter are not in tandem with the modern democratic dispensation."

The pan-African digital rights organisation views the arrest in such a circumstance as a gross violation of O'Donovan's freedom of expression.

O'Donovan, who worked as a manager and media activist at the satirical comedy Magamba TV in Harare, is the first victim of an internet-related offence since Zimbabwe introduced a cybersecurity ministry last month.

The American's arrest has triggered a new phase in the clampdown on social media use in Zimbabwe. The fact that there have been several arrests in recent years for actions deemed to undermine President Mugabe with no convictions so far makes this case exceptional.

In a statement to the Associated Press, Amnesty International's deputy regional director Muleya Mwananyanda said the arrest marks the start of a "sinister new chapter in the Zimbabwean government's clampdown on freedom of speech."