Mr Thackeray says jobs should be given to locals
Violence has broken out in the Indian city of Mumbai (Bombay) after the arrest of the leader of a right-wing regional party.
Raj Thackeray, the leader of the Maharashtra Navnirman Sena, appeared in court on Tuesday on charges of disrupting government work and rioting.
He remains in custody but his lawyers say he will be freed soon.
On Sunday Mr Thackeray's supporters attacked Indian railway recruitment examination centres in Mumbai.
They were alleged to have beaten up job candidates from northern India.
The BBC's Zubair Ahmed in Mumbai says that hundreds of Mr Thackeray's slogan-shouting supporters fought pitched battles with police for hours, causing large-scale disruption in the city.
They demanded the unconditional release of their leader, who was woken up from his sleep and arrested early on Tuesday morning in a town 400km (248 miles) from Mumbai, where he had lined up a series of public meetings.
He was charged with rioting, preventing government employees from performing their duty and damaging property.
There have been protests against Mr Thackeray in northern India
After the news of his arrest spread, his supporters went on the rampage in Mumbai and other cities, smashing taxis and buses.
They hurled stones and broken bottles at riot police, who on several occasions had to run for cover before reinforcements arrived to restore order.
More than 500 people have now been detained in the city.
"The strong arm tactics of the Maharashtra Navnirman Sena (MNS) are not acceptable. The government will not tolerate people taking law into their own hands," Maharashtra's Chief Minister Vilasrao Deshmukh said on Tuesday.
The MNS has been accused of several attacks on migrant workers in recent months.
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.
Analysts say the family split will hit Shiv Sena votes
It is the second time this year that Mr Thackeray has been arrested: he was accused of stoking communal tension in February when he alleged that migrants were "swamping" Maharashtra, India's most industrialised state, in search of jobs.
His supporters in recent months have been accused of attacking north Indian migrants in Mumbai and damaging property associated with them.
While the unrest forced many migrants to leave their neighbourhoods, Mr Thackeray consistently denied inciting it.
Recently his party protested against Jet Airways, India's largest private airline, after it announced that it was laying off 1,900 employees to cut costs.