Matthias Chang was arrested on Thursday under a tough domestic security law that allows detention for up to a month without trial.
Chang had been representing Khairuddin Abu Hassan, a former ruling party member who was detained last month under the same security law after he launched a global campaign to highlight alleged graft by Najib.
Chang released a statement calling his and Khairuddin`s arrests "a heinous tactic" to silence those speaking out "against the tyranny and corruption of the Najib regime", and announced he had begun refusing food.
He is also a former political secretary to Mahathir Mohamad, Malaysia`s premier from 1981 to 2003, who has led calls for Najib to be investigated.
Najib faces mounting calls to explain massive sums of money allegedly missing from a state-owned development company he launched, as well as the sensational revelation in July that nearly $700 million in mysterious transfers had been made to his personal bank accounts.
The pressure grew this week, with Malaysia`s nine state sultans issuing a rare joint statement on Wednesday saying the scandal had created a "crisis of confidence", and calling for a full investigation.
Both Najib and the state-owned development company, 1Malaysia Development Berhad (1MDB), vehemently deny any wrongdoing.
Inquiries have been hobbled, however, with Najib firing his attorney general in July, and police raiding the offices of the country`s anti-corruption agency in August.
Khairuddin, aided by Chang, had visited authorities in several countries to call for investigations into the cross-border money flows.
Regulators in the United States and elsewhere are now reportedly scrutinising the affair, with authorities in Switzerland and Singapore saying they had frozen some accounts as they probe possible money-laundering and other crimes.
Najib`s allies say the money deposited into his accounts was "political donations" from Middle Eastern sources, but have refused to provide details.
Rights and legal groups have harshly criticised the arrests of Khairuddin and Chang under the Special Offences (Special Measures) Act, which Najib`s government had passed with a promise it would not be used against political opponents.
Malaysia Bar Council President Steven Thiru said the bar was "absolutely appalled and outraged" over the arrests.
Detained Malaysian lawyer on hunger strike over Prime Minister Graft Scandal
09 October 2015
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A detained Malaysian lawyer has gone on a hunger strike after being arrested over his involvement in efforts to expose a scandal rocking Prime Minister Najib Razak`s government.
