"We are deeply concerned by violent incidents which started Friday in the Libyan capital, Tripoli, and have left more than 40 people dead and hundreds others injured," spokeswoman Ravina Shamdasani said in a statement. "These are the bloodiest incidents Tripoli has seen since the end of the 2011 conflict."
Libya has struggled to ensure a sense of national security and national unity in the years since civil war ended with long-time ruler Moammar Gadhafi's death in late 2011.
Witnesses to the violence last week said militia opened fire on demonstrators in Tripoli during protests meant to convince armed gangs to leave the capital city peacefully, CNN reported.
Shamdasani said from Geneva, Switzerland, the United Nations was calling in Libyan authorities to ensure justice is served for those found responsible for the unlawful killings last week.
"We urge all sides to exercise the utmost restraint to avoid the escalation of violence and engage in peaceful dialogue," she said.