The Secret Service Saturday vowed to block any plan to grant presidential assailant John Hinckley Jr. a supervised day's outing from the psychiatric hospital where he has lived for the past six years.

St. Elizabeths Hospital proposed a recreational day trip for Hinckley in a letter dated July 19, and hospital officials believe the trip could be the first in a series of such outings for Hinckley, 33, who tried to assassinate President Reagan in 1981.The Secret Service has steadfastly opposed attempts to let Hinckley leave the hospital and said Saturday that this request will not alter its belief that Hinckley remains a threat to the president and the public.

"We are totally opposed to it," said Secret Service spokesman Rich Adams. "Nothing has been shown to us that would make us feel differently. We would not want a man like John Hinckley - with all the damage he has done - walking on the steets. I'm sure the public would feel the same way."

But on Saturday, Gordon Loux, a longtime friend of the Hinckley family who held a dinner for Hinckley during his only unsupervised visit in 1986, said the chief executive's security force is being unreasonable.

"From their perspective, I think they think they need to be cautious, but I think they are being overly cautious," said Loux.

"The Hinckleys are a great family and they've been through a lot of agony. I'm not a psychiatrist . . . but it certainly seems fitting he should be allowed a visit, especially with his family and friends.