China placed the 17.5 million residents of the southern city of Shenzhen into lockdown for at least a week, seeking to halt a growing COVID-19 outbreak with a move that could cause disruption and production delays in the key technology hub and port.

The lockdown, which came after virus cases doubled nationwide to nearly 3,400, will be accompanied by three rounds of city-wide mass testing, according to a government notice. The measure, announced Sunday, followed earlier restrictions placed on Shenzhen’s central business district, and will last until March 20.

All bus and subway systems were shut, and businesses, except those providing essential services, have been closed. Employees were told to work from home if they can. Residents will be barred from leaving the city except in limited situations.