Erica Mendell Daye, whom Prince George's County police have charged with first-degree murder in the decapitation and mutilation of her 5-year-old son, halted her arraignment yesterday in D.C. Superior Court, demanding that she have her own attorney and taunting marshals with racial epithets.

As her court-appointed attorney looked on, Daye halted the hearing examiner, Roy M. Ellis, in midsentence with a series of demands and accusations. "Who is this man, he is not my counsel," Daye shouted, pointing to Abraham Blitzer, her court-appointed attorney. "I haven't been able to make a phone call. I've been held in confinement for 24 hours. I want my legal rights."

Ellis asked her who, if not Blitzer, was her attorney. Daye answered, "Is that important? I don't think that's anyone's business."

Ellis ordered Daye to submit to psychiatric testing, and she is scheduled to go before the court today to decide whether she is mentally capable of understanding the charges.

Daye's bizarre courtroom performance contrasted with the picture of the 25-year-old woman drawn by some friends and neighbors who lived near her at the University Garden Apartments in the 7900 block of 18th Avenue in Adelphi. The seemingly uncontrollable character was also a marked difference to the cool customer convicted of writing a series of bad checks at Prince George's County stores between 1981 and 1983, according to court documents.

And the woman described by friends as a loving, playful and concerned mother is markedly different from the person who, police charged, used a butcher knife to decapitate, dismember and disembowel little William Lawrence Deloach.

"That wasn't her . . .that's not the Erica I know," said a man who, after the hearing, identified himself only as a four-year friend of Daye's. He described Daye as "outgoing, a loving mother. She was a friend to the child; she loved her son as much as she loved herself, if not more."

Blitzer, Daye's court-appointed attorney, said Daye will be evaluated by a psychiatrist at St. Elizabeths Hospital before the rescheduled hearing at 4 p.m. today. Blitzer said Daye was hostile and uncooperative during the time he spoke with her. She is being held without bond.

Police still have no motive in the slaying. According to the statement of charges against Daye, a county police officer went to her second-floor apartment about 4:20 p.m. Saturday to check on a call from Daye's phone number about a suspicious man with a gun in the area. But Daye refused to let the officer inside the apartment, according to the statement, telling him "he would not want to see what was inside."

At 11 a.m. Sunday, according to the statement, Daye went to the Park Road Community Church, 1019 Park Rd. NW, to talk with the Rev. Eugene Burton. "At this time (Daye) appeared to be on drugs and was talking irrationally," the statement said. "When the Rev. Burton asked where (Daye's) son was, Daye replied that 'he is safe now' several times."

Burton arranged for Daye to be taken to Howard University Hospital, the statement said, but Daye left before the hospital staff finished interviewing her.

Reached at home last night, Burton declined to comment about Daye. "It's a tragic case, that's all I want to say," Burton said. "Our hearts and our prayers go out to her and her family."

On Sunday evening, homicide investigators found some of the boy's internal organs wrapped in bags inside the apartment's refrigerator-freezer. Daye's fingerprints were on the bags, police said.

Daye had moved through six jobs since graduating in 1978 from Coolidge High School in Washington, usually working as a receptionist. Her employment included stints at Presearch Inc., a computer software firm in Arlington; Booze-Allen & Hamilton, a consulting firm in Bethesda, and Sallie Mae, a private firm that handles government guaranteed student loans, in Washington. Bill Bowser, vice president of Presearch Inc., said several employes recalled Daye's working there after recent news reports about the killing of her son. "When something like that happens, it kind of becomes a buzz," Bowser said.

Daye resigned from the computer software company in February 1985, the same month that a Maryland probation officer declined to have a warrant issued for Daye's failure to meet the conditions of her three-year probation for a conviction for writing bad checks.

Daye was scheduled to report to her probation officer yesterday.