"We have rounded up hundreds of Tibetan protesters. The number is over 700," said Nawaratna Poudel, a police officer outside the Chinese embassy in Kathmandu -- the scene of almost daily protests.
"This is probably the largest number of detentions in a single day so far," he said, but added they were likely to be released later in the evening.
But Kathmandu police chief Sarbendra Khanal said three senior Tibetan activists were being held "on the charge of being involved in anti-China activities in Nepal."
"They are using Nepal's territory for political purposes and trying to disturb peace and security in our country. It is against the law," he said, adding that police were also searching for several other top activists.
The official gave no further details on what the alleged anti-China activities were.
The leaders detained were from the Tibetan Women's Association, an activist group, and Tibetan Reception Centre, a body which aids refugees after they make the arduous illegal crossing into Nepal from China.
An official from the Tibetan Reception Centre dismissed the charges.
"Police have accused the leaders of being involved in organising anti-China demonstrations and have been given the arrest warrant for 90 days," Dorji Damdul, an official of the centre, told AFP.
"We condemn this act by police as they have been arrested without any proof. Our leaders are not involved in any anti-China activities in Nepal," he said.
Kathmandu has seen almost daily protests since unrest erupted in Lhasa, capital of the Tibetan region, prompting a crackdown by Chinese security forces in March.