"I remind the government of its international obligation to respect the right of peaceful assembly, and I remind its security forces of their obligation to use only minimum necessary force," Louise Arbour, the UN high commissioner for human rights, said in a statement.
Arbour, a Canadian judge, said on Thursday she "was shocked by the excessive use of force by security forces in Nepal, as well as the extensive use of arbitrary detention in violation."
She added that she would give UN peace-keeping officials information her office gathered about anyone implicated in abuses, including those from the Royal Nepalese Army.
Nepal currently has about 3500 soldiers and police in 12 UN peace-keeping missions around the world, making it the fifth largest contributor after Bangladesh, Pakistan, India and Jordan.
Tens of thousands of people have staged protests against Nepal's King Gyanendra over the past week. Three people have died and doctors said hospitals were flooded with patients, some with bullet wounds after clashes with security forces.
Protests against Gyanendra are the most intense since he seized power 14 months ago, citing the inability of the civilian government to resolve the conflict with Maoist insurgents.
Kofi Annan is said to be concerned
about the situation in Nepal
Stephane Dujarric, the UN spokesman, said if Nepalese forces and police were involved in human rights violations "it is something that would be brought up with the peace-keeping operation and followed up on."
The New York-based Human Rights Watch has been urging UN peace-keeping officials for the past year to screen Nepalese troops. It is not clear if any action has been taken.
Dujarric said that Kofi Annan, the UN secretary-general was "more than ever concerned" about the situation in Nepal and was urging a national dialogue of all political forces.
"It is quite clear that the Nepalese people want a swift end to the conflict and instability and immediate restoration of democracy. The loss of life and denial of legitimate rights should end without delay," Dujarric quoted Annan as saying.