CHICAGO — A Chicago ban on gun sales within the city, aimed at reducing gun violence, is unconstitutional because it goes too far in barring buyers and dealers from engaging in lawful sales, a federal judge ruled on Monday.

U.S. District Judge Edmond E. Chang found that the U.S. Constitution’s protection of the right to keep and bear arms must include the right to acquire them, within limits.

The judge stayed the ruling, however, in order to give the nation’s third-largest city a chance to respond. Chang said the city had until Monday, January 13, to submit a motion to stay the ruling pending an appeal if it chooses to do so.

Chicago Mayor Rahm Emanuel “strongly disagrees” with the court’s decision, according to a statement from the city, adding that he has instructed the city’s lawyer to consider all options to better regulate the sale of firearms within the city’s borders.

“Every year Chicago police recover more illegal guns than officers in any city in the country, a factor of lax federal laws as well as lax laws in Illinois and surrounding states related to straw purchasing and the transfer of guns,” the statement said. “We need stronger gun safety laws, not increased access to firearms within the city.”

Gun sales are allowed outside of Chicago, including in neighboring suburbs, and anti-gun activists have complained that criminals use “straw buyers” to buy guns outside of Chicago and bring them into the city.

The court decision comes as Chicago reported about 17 percent fewer murders in 2013 as the city flooded high-crime zones with police. The murder rate in the city was still higher in 2013 than in the larger cities of New York and Los Angeles.

The judge’s ruling was in response to a lawsuit by the Illinois Association of Firearms Retailers.

Chuck Cooper, a lawyer for the plaintiffs, praised Judge Chang’s decision in an email, saying it was a “careful, scholarly ruling upholding the Second Amendment claims in this case.”

Also this month, a law allowing Illinois residents to carry concealed weapons takes effect. Illinois was the last state in the nation to allow residents to carry concealed weapons.