HRW said members of the Shiite community in Tripoli are facing pressure from their Sunni counterparts in the northern city and the official Lebanese response has been weak so far.
"With battles going on in Tripoli and with people being targeted, beaten, knifed and killed, the Lebanese government can't afford to sit on its hands," Joe Stork, deputy Middle East director at Human Rights Watch," said in a statement.
"[The government] needs to start arresting and prosecuting the people behind the violence in Tripoli and confiscate their weapons," Stork said
More than 100 members of a special Lebanese police unit accompanied 480 regular police officers to Tripoli to respond to early December clashes that left at least 12 people dead and 100 others wounded.
In 2005, Lebanon broke free of Syrian dominance with its Cedar Revolution but Syria's civil war is threatening to once again ensnare Lebanon, observers say.
Syrian President Bashar Assad is an Alawite.