Eddie Chu was one of several rebel lawmakers to win seats in landmark legislative elections earlier this month.
He and his family have been under 24-hour protection since the vote on September 4 due to threats to their safety.
Police told AFP today that six men had been arrested for "intimidation" linked to the death threats made to Chu, but gave no further detail.
The South China Morning Post said suspected triad members were among those arrested in Wednesday morning raids, citing an unnamed police source.
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Chu is an environmentalist who has campaigned against the destruction of heritage properties to make way for newer developments and is pushing for better rights for rural residents.
He has sought to expose shady political dealings in rural areas where local strongmen are influential and have been accused of having links to criminal gangs.
Triad gangs have traditionally been involved in drug-running, prostitution and extortion but are increasingly in legitimate ventures such as property and the finance industry.
Some are believed to also have links with the political establishment and there have previously been allegations of triads sending paid thugs to stir up trouble during protests.
Chu has previously said he received the threats because of his political views.
"The threat to my personal safety is imminent," he told reporters two weeks ago outside the police headquarters.
The legislative elections were the first major polls since the 2014 democracy rallies, with the anti-establishment camp increasing its share of the legislature, which is still weighted towards Beijing.
The election saw the highest voter turnout since Hong Kong was returned to China by Britain in 1997 under a handover agreement guaranteeing its freedoms for 50 years.