When the Grateful Dead played Foxboro, Mass. last month, the police called in the Massachusetts National Guard. But it was not because the crowd was rioting.

Instead, the unusual deployment was a new wrinkle in the National Guard's escalating antidrug mission -- assisting local police to identify suspected drug users. Critics charge it shows how civil liberties can quickly be eroded when the military becomes involved in domestic law enforcement.

As with the regular military, the Guard operates under sharp restrictions defined by the 1878 Posse Comitatus Act: National Guard officers have no powers of arrest and function only in "support" of local or federal law enforcement agencies.

But as the Grateful Dead assignment shows, there is still considerable leeway for the Guard to become involved. One of the Guard's assigned missions in the drug war is "aerial and ground surveillance." So when Foxboro Police Chief Edward O'Leary proposed using the Guard to help him cope with an expected influx of drug-using fans at the Grateful Dead concert, Pentagon officials in Washington quickly approved the plan.

While one Army helicopter hovered over the Foxboro stadium, about 40 Massachusetts Guard officers -- equipped with night vision goggles and hand-held radios -- fanned out across the parking lot and stadium looking for suspicious activity. Some of the Guard officers were in uniform, but others wore plainclothes.

"We were able to provide them {the police} with 100 different tips," said Capt. Tammy Miracle, spokeswoman for the Massachusetts Guard. The Foxboro police reported about 30 arrests.

But to critics, it is but one more example of how the drug war is getting out of hand. "This sounds like something out of Bulgaria -- in the old days," said Arnold Trebach, president of the Drug Policy Foundation, a Washington advocacy group that is critical of government drug policies. "What are we going to have next -- Army paratroopers at these events? Why not have tanks set up outdoors? Is there no limit to what this society will do in the name of fighting the drug war?"