The Swedish mother of a 5-month-old girl who drowned in a downtown hotel here April 16 was indicted by a D.C. Superior Court grand jury yesterday on a charge of second-degree murder.

In charging Berit Maria Melin, 29, with killing her child, the grand jury rejected the mother's story to police that a knife-wielding intruder has forced his way into her room at the Capital Hilton and drowned the child when Melin failed to respond to his demands for money. The grand jury indictment alleged that Melin killed the child "with malice aforethought."

According to informed sources, Melin had had an argument ealier in the morning with the child's father, Lars Levin, a Stockholm gem dealer. Levin, who was not present when the child drowned, returned to Sweden last week after his passport, which he had voluntarily surrendered to law enforcement officials, was returned to him.

Melin in being held under tight security in the psychiantric facility at Washington Hospital Center, where she was admitted in April for observation.

According to an imformed source, Melin was suspect from the outset of the investigation because statments made to police concerning the incident conflicted with physical evidence found in the hotel room. In addition, a source said, Melin gave contradictory statements to police concerning the incident.

Principal Assistant U.S. Attorney Carl S. Rauh said the indictment of Melin "concludes this matter" and that no further action on the case is expected by the grand jury.

Swedish embassy spokesman Lars Arro said in April that Levin and Melin came to Washington with their daughter, Lilly-Sophie, on April 13 after stopping in New York, where Levin conducted some business in connection with his gem dealing.

At about 12:30 p.m., on Monday, April 16, while Levin was away from the hotel visiting B'nai B'rith, Melin told police, an intruder armed with a knife forced his way into her fourth-floor room and demanded a large sum of money.

Police said she told them she offered the intruder the small amount of American and Swedish currency in her purse, but he continued to demand a larger amount. To enforce his demand, police said she told them, the intruder took Melin's baby from a crib, turned the water on in the bathtub and threatened to drown the child.

According to a police description of Melin's account, after repeated threats during which Melin said she had no more money, the intruder "placed the baby in the tub" and then left.

Melin, police said, told them she left the room to seek help. She returned to the room with hotel security personnel who found the baby in the tub and called an ambulance. The baby was dead on arrival at Children's Hospital National Medical Center.

Police said the baby had been found nude and floating face down in the tub, which contained about six inches of water.

Melin is expected to be arraigned in Superior Court within a week. CAPTION: Picture, BERIT MARIA MELIN . . . held for observation