Authorities detained a firebrand young leader of the Patel caste in the Indian state of Gujarat as his supporters gathered for a rally.
Hardik Patel, 22, has led huge protests on behalf of the community and had planned to march from Surat to the state capital Ahmedabad.
Mobile internet services have also been suspended across Gujarat.
Mr Patel's movement is demanding quotas in government jobs for the community, despite their relative prosperity.
There were reports of protests in Ahmedabad after Mr Patel was detained along with, Indian media said, 78 other protesters.
He was detained as he gathered with several thousand supporters in Surat for a rally which the government said it had banned.
Later, police used batons and tear gas to disperse a crowd of some 3,000 supporters protesting against the detention in Gujarat's largest city, Ahmadabad, reported AP news agency.
Last month, two days of caste-related violence left eight people dead and the army was deployed to maintain peace.
Gujarat has been particularly alert to incidents of violence since 2002 when religious riots left at least 1,000 people dead, most of them Muslims.
Patels, who comprise some 20% of Gujarat's population, control India's thriving diamond cutting and polishing industry and are among the most prosperous businessmen and farmers in Gujarat.
But they complain that they are denied college places because of affirmative action to ensure those below them in India's complex social structure have access.
And they say they have been affected by a slowdown in small and medium industries in Gujarat which has led to a drying up of employment opportunities.
Patels are now demanding affirmative action and classification as Other Backward Classes (OBCs) so that they are entitled to quotas in state-run education centres and in government jobs.
OBC refers to the castes - in the middle of the Hindu caste hierarchy - who do not face so much exclusion or isolation in society but have been traditionally socially and economically disadvantaged.