Anwar began a 5-year prison sentence yesterday after the country's top court ruled there was overwhelming evidence showing that he sodomized a former male aide.
The case was widely seen as politically motivated to eliminate any threats to the ruling coalition, whose popularity has been eroding after more than five decades of unquestioned dominance.
Police detained cartoonist Zulkiflee Anwar Alhaque, better known as Zunar, at his home late Tuesday over a series of tweets on Anwar's case. A cartoon he posted on Twitter showed Prime Minister Najib Razak as the judge in Anwar's case.
The police moves were criticized by rights groups, with New York-based Human Rights Watch saying it was shameful that Malaysian authorities had turned peaceful criticism into a criminal act.
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"Clearly it is designed to intimidate and instil fear in people on social media to go silent on their views. It is a further erosion of freedom of expression in Malaysia," said the group's Asia deputy director Phil Robertson.
Anwar has been the most vocal and visible symbol of the opposition's resurgence and is seen as the most potent political threat to Najib.
Anwar's second daughter, Nurul Nuha Anwar, today launched a "March to Freedom" campaign, backed by her five siblings. Eldest sister Nurul Izzah is already a lawmaker.
Nuha said the family was devastated and outraged over the "malicious persecution" of Anwar that led to his jailing again.
"These seventeen years have been an emotional roller coaster. We do not know how long this struggle will continue but we will be with our father till the end," said Nuha, 31. Details of the campaign will be announced later.
Anwar was accused of sodomising Saiful Bukhari Azlan, then 23, who was working as a lowly aide in the opposition campaign office in 2008.
Homosexuality is a crime in Malaysia punishable by up to 20 years in prison and by whipping, although prosecutions are rare.