Police in the Russian capital Moscow say they have arrested 236 people suspected of being illegal migrants from Central Asia.
They were detained in an industrial zone. A further 154 Central Asians were arrested in a separate raid, Russian media report.
Russia increasingly relies on cheap labour from Central Asia, where many families depend on migrant earners.
But the issue of migration has fuelled social tensions.
Of the 236 people detained on Dorozhnaya Street, 100 were found to have illegal status, a Russian migration official later told Interfax news agency.
Among items confiscated during the raid were an improvised pistol, five cartridges, two daggers and what appeared to be two stolen cars. Police also said they had discovered an illegal cafe at the site.
The 154 Central Asians detained on Lyublinskaya Street, in the south-east of the city, were found in an abandoned building, police told Interfax, without giving details.
Millions of Uzbeks, Tajiks and Kyrgyz people have migrated to Moscow and other parts of Russia in recent years, in search of work.
Often poorly paid and badly housed, they are predominantly Muslim, creating tensions with central Russia's mainly Orthodox Christian population, and they frequently face discrimination. Moscow's lack of mosque space is a particular sore point.
Russian officials have also voiced concern about the involvement of migrants in crime, such as the trafficking of heroin from Tajikistan. On Wednesday, 80kg of what appeared to be heroin were found aboard a train travelling from the Tajik capital, Dushanbe, to Moscow, Russian police said.