Two people were killed on Friday and at least 112 injured, including 77 security officials, authorities said, after police used force to stop the stone-throwing crowd from marching towards the parliament building in the capital, Kathmandu.
Protesters vandalised homes, shops, a hospital, a political party office, vehicles and a shopping mall and snatched a weapon from the police, the authorities in the Himalayan nation said.
"This is sheer vandalism, arson, looting and anarchy. It cannot be a protest," cabinet spokesman Prithvi Subba Gurung, the minister of communication and information technology, told Reuters.
Rishiram Tiwari, Kathmandu's chief district officer, said 105 protesters including some pro-monarchy leaders were taken into custody.
Nepal's 239-year-old monarchy was abolished in 2008 by a specially elected assembly as part of a deal with Maoist former rebels, ending an insurgency that killed 17,000 people between 1996 and 2006.
The last king, 77-year-old Gyanendra, lives as a commoner with his family in a private house in Kathmandu.
Political instability has rocked Nepal, one of the world's poorest countries, with 14 governments since the abolition of the monarchy, hampering economic growth and discouraging investment.
Public frustration has risen over the failure of successive governments to deliver on commitments to develop the economy of the country, a natural buffer between Asian giants China and India.
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