Since when have nonwhite urban neighborhoods have been subjected to heightened surveillance and police repression?

I believe it was since the racial segregation.
During this period, the law enforcement believed that most criminals run over poor neighborhood (which mostly populated by minorities)
These claims however only account for blue collar crimes while ignoring the white collar crimes that mostly conducted by people in wealthy population.

The 'nonwhite urban neighborhoods' have been subjected to 'heightened surveillance' and 'police repression' since the 1960s.

Explanation:

The racial uprisings that occurred during the 1960s substantially resulted in the increased crime rate against the whites. In order to decrease the offenses committed by the nonwhite urban neighborhoods and to keep a check on their activities, the Federal Government chose to heighten the surveillance and allow police repression. Though this decision of the government is seen to have been successful in bringing down the crime rate, it is still criticized by many.