PROVIDENCE, R.I. — An old police tradition of requiring off-duty officers to carry their weapons — ‘‘always armed, always on duty’’ — is being scaled back in police departments nationwide, increasingly being blamed for the deaths of officers shot by colleagues who thought they were criminals.

The policy requires officers to respond to crimes even when they’re not on duty. Supporters also say that letting officers carry their guns off-duty protects them from crooks bent on revenge.

But critics point to the shooting of officers in Providence, R.I., Orlando, Fla., Oakland, Calif. and elsewhere.

The policy is at the center of a $20 million civil rights lawsuit being heard this month in Providence, where Sgt. Cornel Young Jr. was killed in 2000 while he was off duty and trying to break up a fight. He was dressed in baggy jeans, an overcoat and a baseball cap, and carrying a gun.

‘‘Our situation is the extreme example of what can go wrong,’’ said Sgt. Robert Paniccia, president of the Providence police union.

Young’s mother, Leisa Young, says the rookie officer who shot him was not adequately trained to recognize off-duty or plainclothes officers.

The International Association of Chiefs of Police has called ‘‘always on duty’’ policies a costly tradition.