Zimbabwean police have arrested a US citizen for allegedly "insulting" President Robert Mugabe in a tweet she shared, her lawyers say, in the first apparent arrest since the country's creation of the Ministry of Cyber Security.

Martha O'Donovan, who works for Magamba TV, was arrested on Friday during a dawn raid on her home in Harare, the television station said in a statement.

She had re-tweeted a post which apparently referred to Mugabe as a "selfish man and sick man", Zimbabwe Lawyers for Human Rights (ZLHR), which is representing O'Donovan, said in a tweet on Friday.

Zimbabwe Republic Police "says Martha O'Donovan insulted Mugabe when she allegedly tweeted that 'We are being led by a selfish man and sick man'", ZLHR tweeted. The post shared by O'Donovan did not refer to Mugabe by name.

ZLHR also said on Twitter that O'Donovan is being charged with "undermining [the] authority of or insulting [the] president".

#FreeMartha #ZRP says @Martha_ODonovan insulted #Mugabe when she allegedly tweeted that "We are being led by a selfish man and sick man." — ZLHR (@ZLHRLawyers) November 3, 2017

'Fishing expedition'

Doug Coltart, a human rights lawyer in Harare, capital of Zimbabwe, said that if convicted, O'Donovan could face more than 20 years in prison.

"O'Donovan is charged with insulting the president, which is punishable with a sentence of a year," he told Al Jazeera.

"More significantly, she is charged with subverting a constitutional government, which carries a hefty sentence of 20 years in prison. It is almost equal to treason," Coltart said.

"This is directly related to her role with Magamba TV, and a subsidiary of Magamba called Open Parly, which talks all about the work of parliament.

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"This is what we expected all along, that it was not really about the retweet. It was really a fishing expedition to get information about her work at Magamba TV, and try and go after the organisation itself."

'New battleground is social media'

This is the first arrest since the creation of the Ministry of Cyber Security in Zimbabwe last month.

The new ministry focuses on crimes on social media and the worldwide web in general, in advance of the 2018 presidential election.

Amnesty International condemned the arrest, saying it "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".

The human rights organisation added that the charges "confirm fears that this new portfolio [ministry] will simply be used to punish anyone speaking out against the authorities on social media platforms".

Zimbabwean officials did not immediately respond to requests for comment.

Mugabe has been ruling Zimbabwe since its independence from Britain in 1980.

He says he seeks to run for another five-year term in next year's elections.