Twenty-three incidents in which needles and similar objects have been found since February in food sold at two area supermarket chains were reported yesterday by Washington police and supermarket spokesmen.

No injuries were reported in any of the incidents, five of which occured in a Giant store and the others in Safeway stores. All but one of the incidents occured in the District of Columbia.

Police held a press conference yesterday concerning the incidents and said an investigation is under way.

Spokesmen for the chains said they had no idea who was involved in inserting the objects in the food products or why it was being done.

At Giant the items involved included bread and other baked goods and in one instance a package of prewrapped hot dogs, according to a chain spokesman.

The incidents at Safeway involved a similar mix of product, as well as pork chops, a company spokesman said.

The Giant spokesman said all the incidents at the Giant store, which was not identified, involved heavy duty sewing needles of from 1 1/2 to 2 inches in length.

In addition to needles, pieces of wire figured in one of the incidents at the five Safeway stores involved, none of which was identified. There was also an incdent in which glass was found in a jar of margarine, a spokesman said.

Police said customers "should look closely" at wrapped food packages and if questions arise should return them to store personnel for further examination.

"They (the chains) and we are doing everything we can," said a police spokesman.

Tony Statom, a public relations representative for Safeway, said store personnel have been asked to examine packages that appear to have been tampered with, and have found several of the incidents of constamination.

Police said the first of the incidents occured Feb. 17 at a Safeway Store. The most recent apparently occured at a Giant Thursday.

Giant said surveillance has sbeen increased at a all stores and electric equipmentt is being used to search for contaminants. Giant said one of the five incidents at its store was reported by store personnel, the others by customers.

Police said they staked out a chain store this morning as part of the investigation, but made no arrest.