UN High Commissioner for Human Rights Michelle Bachelet is "concerned by the ongoing events in the Hong Kong Special Administrative Region and the escalation of violence that has taken place in recent days," her spokesman Rupert Colville told reporters in Geneva.
The UN rights chief, he said, was calling for a "prompt, independent, impartial investigation" into alleged excessive force by police against the protesters.
The comments came as pro-democracy protesters blocked passengers at Hong Kong's airport Tuesday, causing further travel chaos a day after triggering an unprecedented shutdown.
The 10 weeks of unrest, which has seen millions of people take to Hong Kong's streets, has become the biggest challenge to Chinese rule of the semi-autonomous city since its 1997 handover from Britain.
The protests began in opposition to a bill that would have allowed extraditions to the mainland, but quickly evolved into a broader bid to reverse a slide of rights and freedoms in the southern Chinese city.
Colville said the UN rights office had reviewed "credible evidence of law enforcement officials employing less-lethal weapons in ways that are prohibited by international norms and standards."
He pointed for instance to the firing of "tear gas cannisters into crowded, enclosed areas and directly at individual protesters on multiple occasions, creating a considerable risk of death or serious injury."
The UN rights office, he said, was urging Hong Kong authorities to investigate all such incidents "immediately".
Authorities should also "ensure security personnel comply with the rules of engagement, and where necessary, amend the rules of engagement for law enforcement officials in response to protests where these may not conform with international standards," he said.
Bachelet, he said, also condemned any violence or destruction of property by protesters, and "urges everyone participating in the demonstrations to express their views in a peaceful way."
"She calls on the authorities and the people of Hong Kong to engage in an open and inclusive dialogue aimed at resolving all issues peacefully," he said.
"This is the only sure way to achieve long-term political stability and public security by creating channels for people to participate in public affairs and decisions affecting their lives," Colville said, stressing that the right to freedom of expression and peaceful assembly are enshrined in the Universal Declaration of Human Rights and in Hong Kong law.