UKIAH, Calif. — An intensifying federal crackdown on growers and sellers of state-authorized medical marijuana has badly shaken the billion-dollar industry, which has sprung up in California since voters approved medical use of the drug in 1996, and has highlighted the stark contradiction between federal and state policies.

Federal law classifies the possession and sale of marijuana as a serious crime and does not grant exceptions for medical use, so the programs adopted here, in 15 other states and in the District of Columbia exist in an odd legal limbo. While federal agencies have long targeted Californians who blatantly reap illegal profits in the name of medicine, or who smuggle marijuana across state lines, the Justice Department said in 2009 that it would not normally pursue groups providing marijuana to sick patients, in accordance with state laws.

But in the last several weeks, federal prosecutors have raided or threatened to seize the property of scores of growers and dispensaries in California that, in some cases, are regarded by local officials as law-abiding models. At the same time, the Internal Revenue Service has levied large, disputed tax charges against the state’s largest dispensary, threatening its ability to continue.

In a hint of the simmering federal-state tensions, Kamala D. Harris, the attorney general of California, described in pointed terms the Oct. 7 announcement by four United States attorneys of their tough new campaign against many dispensaries, which they called commercial operations that violate the intent of California law as well as federal statutes.