A 24-year-old Maryland man admitted and then denied to police that he took part in the brutal beating of Ronand J. Pettine at the Iwo Jima Memorial in 1976 that left the young political aide dead, an Arlington Circuit Court jury was told yesterday.

In a statement Alan J. Arnone allegedly gave police shortly before his arrest, he said, "I kicked him (Pettine) once or twice. He was still screaming. We were all drunk that night . . . the police came and we took off running . . . We all had a little bit of blood on us."

That statement was read yesterday by U.S. Park Police Det. Richard LeBrun in the opening day of testimony in Arnone's trial on muder and robbery charges.

Arnone later denied in a written statement to police that he participated in the fatal beating of Pettine. LeBrun also read yesterday from that statement, quoting Arnone as saying "me, myself, was not involved" in the beating.

Pettine, a former campaign aide to Rep. Morris Udall, was found bludgeoned to death on Oct. 2, 1976, in the woods near the memorial, known as a meeting place for homosexuals. Pettine's nude body had been beaten and his sexual organs abused, according to court testimony.

Two men, Michael G. Simoneau, 18, of Falls Church, and Charles A. Baman, 22, of Arlington, already are the slaying.

Arnone has pleaded not guilty to the charges. His lawyer, William D. Dolan, said yesterday in his opening argument that "the youngster" is a victim of "multiple birth defects" who was guilty only of "wanting to find somebody to tag along with."

According to testimony at Simoneau's trial last year, the trio had gone to the Iwo Jima site "to smack around a few queers."

LeBrun testified yesterday that he went to Arnone's home on Jan. 16, 1978.

According to LeBrun, Arnone "stated he was there during the offense." Reading form notes taken during the oral confession, LeBrun quoted Arnone as saying, "We went into the woods. One guy walked up to us. Chucky was pretending like he was queer. He hit the guy with a stick."

All three men, according to Arnone's original statement, participated in the beating, LeBrun said.

Reading from the written statement, given later, LeBrun quoted Arnone as saying: "As soon as the queer approached Chucky (Bamman), that's when it all began. Chucky was drunk and started hitting him and kicking him. But me, myself, was not involved. We (Simoneau and Arnone was just witnesses."

Two men testified yesterday that they encountered Pettine in the woods and engaged in sexual activity with him.

The witnesses said they saw two men walk into the area, one of them carrying a large stick. When Pettine walked over to the other men, the witnesses said they saw Pettine being beaten and heard him scream, "Help me, help me. They're going to kill me."

Under cross-examination by Arnone's attorney, both witnesses said they did not see Arnone, who is under five feet tall.

Referred to as "Big Al," Arnone sat at the defense table yesterday in an oversized jacket, his feet barely reaching the floor.

The jury of seven women and five men seemed visibly disturbed by the graphic testimony. Introduced as evidence by Assistant Commonwealth's Attorney Kenneth Melson were photographs of Pettine's nude, battered body, his torn and bloodstained clothing and the heavy tree limb used to strike the fatal blow.

Simoneau is serving a 35-year sentence in connection with the case. Bamman, accused of strking Pettine with a tree limb and fracturing his skull, pleaded guilty to first-degree murder and received a 40-year sentence.

Both men were sentenced to serve 10 years concurrently for attempted robbery.

The trial is scheduled to continue today. CAPTION: Picture, RONALD J. PETTINE . . . killed near Iwo Jima memorial