In a mailing to thousands of Maryland residents, a national antigun control lobby has attacked the Baltimore County police chief and called for his firing because of his activities against gun legislation that is scheduled to be debated today on Capitol Hill.

"We must act immediately to stop a gun grabber here in Maryland," says the "Urgent Message-Gram" mailed to about 10,000 Maryland residents by the Citizens Committee for the Right to Keep and Bear Arms. It called the police chief, Cornelius Behan, "a New York City import" who "is hard at work undermining your gun rights -- and we must act quickly to stop him."

The Bellevue, Wash.-based lobby, which claims a national membership of 500,000 members, urged its supporters to use "Bounce Behan" bumper stickers and to send post cards to Baltimore County Executive Donald P. Hutchinson asking him to dismiss Behan.

The group, which is working with the Maryland and D.C. Rifle & Pistol Association, singled out Behan because it said he "used tax funds to lobby against gun ownership, misled Maryland's congressmen, worked to turn our police force against gunowners and spread misinformation to the press."

Joe Friend, executive director of the Citizens Committee for the Right to Keep and Bear Arms, said that during a roll call of his officers Behan showed a videotape critical of the McClure-Volkmer bill in Congress. The bill would weaken existing gun control laws.

Behan acknowledged that he made a videotape and showed it to his police force, but said it was for educational purposes. "I certainly made the tape, and I did it as a training tool to acquaint my people with the provisions of McClure-Volkmer," said Behan, 61, who has been the county police chief for eight years and a police officer in Maryland and New York for 39 years.

"I did it to offset the argument put out by the National Rifle Association that all the rank-and-file police officers supported their bill," added Behan. "We knew that any officer that had knowledge of what the bill meant could not and would not support it. Twenty thousand people die every year from handguns. We don't need any legislation that makes guns more available."

In recent months, most of the nation's law enforcement groups have opposed the bill, and one of Behan's Maryland colleagues was quick to defend his actions. He "has taken the leadership for a lot of us chiefs," said Bernard D. Crooke, the Montgomery County police chief. "He's fighting a bill that's not good for the public safety of this country."

"Behan is probably as progressive a police chief as there is in this country," added Crooke. "He is a born leader, and is totally professional. He is respected throughout the state, not only for his positions on issues, but his willingness to take a chance."

The bill, pushed by the National Rifle Association and endorsed by the Reagan administration, proposes the first major change in gun control laws in 17 years by allowing interstate handgun sales and by weakening the 1968 Gun Control Act, which was passed after the assassinations of Martin Luther King Jr. and Robert F. Kennedy.