In February 2008 Raj Thackeray led a violent movement against the dominance of migrants from the North Indian states of Uttar Pradesh and Bihar in Maharashtra and more so in its commercial capital of Mumbai.
In July 2008, Raj issued a public warning that shops in Mumbai needed to have Marathi signboards in addition to the existing English signboards. He warned that after one month, MNS workers would start blackening non-Marathi signboards. While there had been a law to this effect passed earlier by Brihanmumbai Municipal Corporation, it had not been enforced. In September 2008, MNS workers resorted to blackening signboards, to enforce the demand, after which most shop owners complied. Six MNS workers were arrested but later released on bail.
In October 2008, Jet Airways laid off 800 temporary workers and announced layoffs of an additional 1100 workers. The laid-off workers included Marathi as well as North-Indians. Thackeray was asked to intervene. Following this, Raj Thackeray declared that most of these workers had paid security deposits to the company, and he would meet Jet management to plead their case. Unless Jet Airways cancelled the layoffs, his party would not allow any Jet Airways plane to take off from any airport in Maharashtra. Within 12 hours of Raj Thackeray's declaration, Jet chairman Naresh Goyal reversed the layoffs and reinstated the sacked employees. He said that he did it on his own and that there was no political pressure on him.
Raj reproached North Indian leaders for politicising Chhath Puja, a festival popular in eastern Uttar Pradesh, Bihar, and Jharkhand calling it a "drama" and a "show of numerical strength". He stated that the Chhath Puja was a political gimmick by some parties to attract the north Indian vote. He demanded that they only celebrate Maharashtra day and not it in Maharashtra. A petition was filed in the Patna civil court on 8 February against him for his remarks. His statements were criticised from political leaders, especially those from the North Indian states. He accused migrants of swamping Maharashtra, India's most industrialised state, in search of jobs. The MNS chief also accused migrants of disrespecting the local culture. In 2008, expressing his stance on new migrants settling in Mumbai, Raj said, "New immigrants to the city should be denied entry into the city, while those already staying here should show respect to the Marathi 'manoos' and his culture".