Nepali police fired tear gas to break up several thousand demonstrators pressing the king for democratic reforms on Saturday, and held more than 500 people for defying a ban on protests, witnesses said.
Nepal, already battling a Maoist insurgency, has seen some of the biggest rallies in almost 15 years as politicians press King Gyanendra to replace his appointed cabinet with an all-party government, and call elections.
Opposition spokesmen said those detained in the protests near Gyanendra's palace included senior politicians.
Riot police dragged away protesters, who threw stones and damaged vehicles as they were hauled off to makeshift detention centres.
"Police fired teargas and arrested more than 500 protesters, who are being kept in inhuman conditions in a state warehouse," Nepali Congress party official Krishna Sitaula told Reuters.
A day earlier police had held almost 1,000 over a similar demonstration, but released most of them after a few hours.