The detention of Pema Tseden sparks fears over habitual violence by Chinese police against Tibetans.
He had evaded capture for three years after taking part in protests in Ngaba in 2008.
They are taken into custody without explanation a day after police assault other protesters, injuring eight.
They had resisted government orders to tear down 'illegal' shops and homes built near the lake.
Chuchen county residents fear mining may resume after briefly being stopped, though.
Only women take part in the march in hopes of avoiding attack by police.
He had founded an association dedicated to promoting education among Tibetan nomad youth, a Tibetan source says.
Their talks are held behind closed doors and away from the press to minimize offense to China, which objected to the meeting.
Chinese security forces watch the crowd but do not interfere, local sources say.
Chinese authorities try again to reduce the population of Serthar county's Larung Gar Buddhist Academy.
Lobsang Tsering was enrolled in restive Kirti monastery, the scene of frequent protests against Chinese rule.
Villagers oppose Chinese gold-mining operations on a nearby sacred mountain.
Property owners had applied for permission to build, but were turned down after failing to pay bribes, sources say.
The assault follows other raids last year against 'illegal' structures financed and built by nomads on their own land.