More people were arrested last year in New York City on charges of marijuana possession than during the entire 19-year period from 1978 to 1996, according to an analysis released this morning by the Drug Policy Alliance.

Last year, the sixth year in a row that marijuana possession arrests increased, 50,383 people were arrested, according to a report recently released by the New York State Division of Criminal Justice Services and obtained by the policy alliance, which advocates for reform of drug laws.

The figure adds up to 140 arrests a day, making marijuana possession the leading reason for arrest in the city, and represents an 8 percent increase over 2009 and a 69 percent increase since 2005, the alliance reported in a statement issued Thursday.

From 1978 to 1996, there were 49,326 marijuana possession arrests, according to an analysis for the alliance done by Harry Levine, a sociology professor at Queens College and an expert on marijuana enforcement.

The increased enforcement, the policy alliance says, is due not to increasing consumption, but to harsher enforcement.

“Over the last 20 years, N.Y.P.D. has quietly made arrests for marijuana their top enforcement priority, without public acknowledgment or debate,” the group said.

There have been about 350,000 arrests for marijuana possession since Mayor Michael R. Bloomberg took office in 2002, the policy alliance said.

Seventy percent of those arrested are younger than 30, and 86 percent are black or Latino, even though, according to the Drug Policy Group, “young whites use marijuana at higher rates.”

Possession of less than 25 grams of marijuana has been a violation, not a jailable crime, in New York since 1977. But having the drug “open to public view” is a crime, and advocates say that many people who simply have marijuana in their pockets are charged with having it in the open after officers order them to empty their pockets.