WASHINGTON — One of the least restrictive marijuana laws in the country went into effect in the shadow of the White House on Thursday, eliciting stern warnings from the local police but good cheer from many Washingtonians.

“A ticket when you just have a jay or something?” said Clifford Gray, a lifelong District of Columbia resident who is in his 20s, using a slang term for a marijuana cigarette. “I’m good with that.”

Mr. Gray, who spoke outside a subway stop in the Columbia Heights neighborhood, two miles from the White House, was reacting to a new District of Columbia law that reduces the penalty for having up to an ounce of the drug to a $25 ticket. The offense is now a civil infraction, like littering, which carries a fine three times as high.

An ounce can be the equivalent of dozens of marijuana cigarettes. Possession of more than an ounce remains a crime — an acknowledgment that drug dealers are more likely than recreational users to be carrying that much — and carries a maximum penalty of six months in jail and a $1,000 fine. Until Thursday, that same penalty applied to possession of an ounce or less.