Nepalese police have rounded up scores of opposition activists in clashes on the streets of the capital.
Activists object to the king's absolute power
Security forces fired tear gas as pro-democracy protesters hurled stones and scuffled with officers, in protest at King Gyanendra's absolute power.
The government says nearly 200 people were taken away in trucks, but the opposition put the number much higher.
The clashes followed a crackdown on activists on Friday, when protests were stymied by a curfew and mass arrests.
Hundreds of activists were held, while five opposition leaders, including former Prime Minister GP Koirala, were placed under 90 days' house arrest. The government says their freedom of movement could have an adverse effect on peace and security.
While demonstrations in Kathmandu are sometimes good-humoured, Saturday's rally had an atmosphere of bitterness, the BBC's Charles Haviland says from the city.
The activists emerged in small groups to confront security forces, in defiance of the new ban on rallies.
"Down with autocracy" chanted protesters, before fleeing down Kathmandu's alleys in a bid to escape police charges.
Police swiftly pounced on some, manhandling and sometimes beating them - and some ordinary passers by - rounding them up and taking them off in police vans.
In what our correspondent says was a rare development, armed soldiers were brought out, but did not intervene.
Protesters hurled torrents of bricks and stones from many directions, and numerous canisters of tear gas were fired in return, sending people hurrying for shelter up the steps of the picturesque pagoda temple lining the central palace square.
Running battles continued for several hours in side-streets and alleys.
The Associated Press reported earlier that 50 people had been injured, including six police officers.
The opposition wants its supporters to boycott local elections next month, which it says are undemocratic and aimed at entrenching the rule of King Gyanendra, who seized absolute power in February 2005.
Nepal's authorities said they had information that Maoist rebels were planning to infiltrate Friday's rally and incite violence.
There was strong condemnation of the arrests from the international community.
"I call on the king urgently to release those arrested, and to find ways to resume dialogue with the political parties," British junior Foreign Office Minister Kim Howells said in a statement.
There has been an upsurge in violence since the Maoists abandoned a four-month unilateral truce earlier this month.
More than 12,000 people have died in Nepal since the Maoists began their fight for a communist republic 10 years ago.
Six police were killed during a Maoist rebel attack in the western town of Nepalganj on Friday.