Data from the Catalan health ministry showed there were 3,706 cases in the Lleida region on Friday, up from 3,551 the previous day.
The area is dotted with fruit farms where thousands of seasonal migrant workers try to find work.
The pandemic may have slowed across much of continental Europe, but amid dozens of small clusters popping up across Spain, those among these migrant workers are proving particularly worrying for health authorities.
The lockdown was ordered after health authorities detected nine localised clusters of COVID-19, four of which were detected at fruit farms in the area.
In the town of Fraga, where fruit processing plants surround lush peach orchards, 360 infections have been found over the past two weeks.
Under the lockdown order, residents will be allowed to go to work but from Tuesday workers entering or leaving the area will have to show a certificate from their employer.
A field hospital has been set up in Lleida to help treat those who have tested positive for coronavirus.
Alba Verges, the Catalan health spokeswoman, advised elderly people to stay inside to reduce the risk of infection.
At least 200 police have been deployed in the area to make sure residents comply with the lockdown.
One of the worst-hit nations by the epidemic in Europe, Spain has registered 205,545 cases of coronavirus and 28,385 deaths.
Spain’s left-wing government ended its three-month lockdown on 21.
Barcelona’s Sagrada Familia basilica reopened on Saturday, giving frontline workers the chance to have the usually tourist-packed landmark to themselves in recognition of their efforts during the coronavirus pandemic.
The world famous building, designed by architect Antoni Gaudi, closed almost four months ago. But for the next two weekends it will be open to essential workers, including those in healthcare, the police and NGOs, who will be able to explore without the usual crowds.