Late on Tuesday, police detained 529 people, including top political party leaders, in Katmandu after about 6000 people rallied to demand that King relinquish the absolute power he seized in February.
Most were released on Tuesday night or Wednesday morning, police said.
"We have not been deterred by the extreme use of force. We held a meeting last night and decided we will continue our street protest," Ram Sharan Mahat of Nepali Congress, the largest political party, said on Wednesday.
The party heads an alliance of Nepal's seven main political parties which are demanding that the king hand power back to an elected government.
About three dozen people were hurt, some with head injuries, when police beat them with bamboo batons in an attempt to disperse the crowd in the centre of the capital.
King Gyanendra faces demands
to relinquish absolute power
Police dragged hundreds of protesters to police vans and trucks and drove them to detention centres.
Protesters hurled rocks and bricks at police officers, injuring two.
One witness said that a radio journalist covering the event was also beaten up by police.
The rally moved into the city centre despite a government ban on gatherings there.
Protesters broke through police lines, chanting slogans against the royal government.
"Down with autocracy. We want democracy," the protesters shouted.
Activists have held daily rallies
in Kathmandu since last week
"Give back power to the people."
Activists have held daily rallies in Katmandu since last week to protest Gyanendra's seizure of power.
Tuesday's rally was the largest and was led by top opposition politicians.
The king has said he assumed absolute power to end government corruption and to quell an anti-monarchist communist insurgency, which has claimed more than 11,500 lives in nine years.