WASHINGTON — Attorney General Jeff Sessions has ordered federal prosecutors to pursue the toughest possible charges and sentences against crime suspects, he announced Friday, reversing Obama administration efforts to ease penalties for some nonviolent drug violations.

The drastic shift in criminal justice policy, foreshadowed during recent weeks, is Mr. Sessions’s first major stamp on the Justice Department, and it highlights several of his top targets: drug dealing, gun crime and gang violence.

In an eight-paragraph memo, Mr. Sessions returned to the guidance of President George W. Bush’s administration by calling for more uniform punishments — including mandatory minimum sentences — and instructing prosecutors to pursue the harshest possible charges. Mr. Sessions’s policy is broader than that of the Bush administration, however, and how it is carried out will depend more heavily on the judgments of United States attorneys and assistant attorneys general as they bring charges.

The policy signaled a return to “enforcing the laws that Congress has passed,” Mr. Sessions said Friday at the Justice Department, characterizing his memo as unique for the leeway it afforded prosecutors.