The accused, Paresh alias Raju Chougule (27), who is a cousin of Bhiwandi BJP MLA Mahesh Chougule, was arrested yesterday in connection with the case, Thane police PRO Inspector Sukhada Narkar said.
Five others were arrested on Monday in connection with the case, she said.
RPI activist Vicky Dhepe (20) and a few others were assaulted by a group of youth armed with iron rods and choppers on May 11.
Dhepe died on May 15, following which RPI activists went on a rampage.
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The activists had also vandalised two offices of the MLA, they said.
Subsequently, more than 20 people, including Raju Chougule, were booked on charges of murder, attempt to murder, rioting and also under sections of SC/ST (Prevention of Atrocities) Act.
Following his arrest yesterday, Chougule was produced in a court which remanded him in police custody till May 23, Narkar said.