Police fired several rounds of tear gas and used bamboo batons to beat back about 5000 protesters shouting "we want democracy", and stopped them from marching towards the main city centre, an area in Kathmandu where protests are banned.
Girija Prasad Koirala, a former prime minister and president of the Nepali Congress, the country's largest political party, was among those detained by the police.
Koirala, 80, was slightly hurt in the scuffle with police.
He momentarily lost consciousness and was taken to a hospital by the police.
Doctors said his injuries were not serious.
Former PM Girija Prasad Koirala
was hurt in a scuffle with police
"We condemn the use of such force on a peaceful protest. We will continue our protests until democracy is restored," said Arjun Narsingh, spokesman for the Nepali Congress.
Leaders and supporters from seven major political parties had gathered in the Basantapur area in Kathmandu and began marching towards the main market area when riot police charged at them.
A few hundred managed to break the police line but were quickly pushed back, and several rounds of tear gas were fired at them.
Several protesters were injured.
Many of them were brought to the Bir hospital in Kathmandu.
One of the wounded who identified himself as Madhav Poudel said many of his colleagues were injured and that police had beaten him up with a baton.
King Gyanendra took over
absolute power on 1 February
Since Gyanendra took over absolute power on 1 February, there have been several protests against his royal administration, and demands that democracy be restored in Nepal.
The monarch said he was compelled to take over power to quell a communist insurgency that has claimed more than 11,500 lives, as well as to control corruption in the administration.