Police in the Indian city of Mumbai (Bombay) have arrested the leader of a right-wing regionalist party on charges of stoking communal tension.
Mr Thackeray is a nephew of Shiv Sena leader Bal Thackeray
Maharashtra Navnirman Sena chief Raj Thackeray was freed on bail after a two-hour court hearing.
Mumbai police also arrested Abu Azmi, a leader of the rival Samajwadi party. He too was freed on bail.
Activists from Mr Thackeray's party are accused of attacks on migrant workers from northern India.
Mr Thackeray is a nephew of Bal Thackeray, the founder of the right-wing Hindu Shiv Sena party.
The Shiv Sena has long argued for greater rights to be given to ethnic Marathis - people who can trace their origins to the state of Maharashtra, of which Mumbai is the capital.
Raj Thackeray left the Shiv Sena in December 2005 and formed Maharashtra Navnirman Sena in March 2006.
He is believed to have quarrelled with his cousin, Uddhav, the executive president of Shiv Sena and Bal Thackeray's son.
Security has been tightened near Shivaji Park in central Mumbai where Mr Raj Thackeray lives and has his office.
Sena Bhavan, headquarters of the Shiv Sena party, is in the same area and has also been placed under heavy guard.
After his arrest, Mr Thackeray was taken in a convoy of vehicles to a court in Vikhroli.
Abu Azmi was arrested in a simultaneous police operation at his residence in south Mumbai.
The local branch of Mr Azmi's Samajwadi party draws much of its support from people who migrated to Mumbai from the impoverished northern Indian state of Uttar Pradesh.
The two arrests ended a 10-day period of intense speculation, sparked by remarks by Mr Thackeray, criticising migration from northern India.
The Navnirman Sena leader accused migrants of swamping Maharashtra, India's most industrialised state, in search of jobs.
Following his statement, his supporters attacked north Indian migrants in Mumbai and damaged property associated with them.
Mr Thackeray has denied inciting violence.
The unrest has forced many migrants to leave their neighbourhoods.
A taxi driver from Uttar Pradesh accused the leader of trying to "expel a community".
"It is not possible to stop anyone from coming here. If I had a good job in my village in Uttar Pradesh why would I come here?"