SAN FRANCISCO — The City of Oakland has filed a lawsuit in federal court to prevent the Department of Justice from seizing property leased to the largest medical marijuana dispensary in the country.

“This lawsuit is about protecting the rights of legitimate medical patients,” City Attorney Barbara Parker said in a statement on Wednesday, when the suit was filed. “I am deeply dismayed that the federal government would seek to deny these rights and deprive thousands of seriously ill Californians of access to safe, affordable and effective medicine.”

The civil lawsuit, which the City Council approved, seeks to “restrain and declare unlawful” the forfeiture proceedings against the landlords of the dispensary, Harborside Health Center, stating that Oakland will “suffer irreparable harm if the dispensaries are shuttered.”

“It is heartening to see the city stand up and support us,” said Steve DeAngelo, Harborside’s executive director. At its Oakland location, the nonprofit dispensary employs 100 people and serves some 112,000 more, seeing 600 to 800 customers a day. Last year, the group paid $3.5 million in taxes, including $1.1 million to the city.

“Oakland is very cash strapped,” said Cedric C. Chao, a lawyer for the city. “People can do the math. How many police officers is that? How many teachers?”

Oakland is the first city to file such a lawsuit amid escalated efforts by federal prosecutors to close the state’s dispensaries. Since last October, the United States attorneys in California have sent hundreds of letters to dispensary owners, landlords and local officials, threatening forfeiture of assets and criminal charges.

The Department of Justice has focused its efforts on those dispensaries near schools, parks and playgrounds. Harborside’s two dispensaries have no such neighbors, but in July, United States Attorney Melinda Haag issued a forfeiture notice on the 7,000-square-foot Oakland property leased to Harborside since 2006 and on a 6,000-square-foot property occupied by the group’s San Jose dispensary.

In a statement, Ms. Haag called Harborside an example of a marijuana “superstore.” She went on to say that the larger the dispensary, “the greater the likelihood that there will be abuse of the state’s medical marijuana laws, and marijuana in the hands of individuals who do not have a demonstrated medical need.”

Though Harborside’s landlords in both locations moved to evict after receiving notice from the United States attorney, the dispensary has challenged the evictions in court and both locations remain open.

Under federal law, all possession and distribution of marijuana is illegal. But in 1996, California became the first state to legalize medical marijuana. Now, 17 states and the District of Columbia have medical marijuana laws.

Some 500 dispensaries across California have closed in the past year as a result of pressure from federal prosecutors and approximately 1,000 remain open, according to Kris Hermes, a spokesman for Americans for Safe Access, a nonprofit medical marijuana advocacy group.

“Whether or not this lawsuit is successful,” Mr. Hermes said, “it represents an important push back by local officials who are tired of being bullied by the federal government.”