WASHINGTON (AP) _ Evidence seized during a raid on the headquarters of political extremist Lyndon LaRouche contains numerous references to the slaying of Swedish prime minister Olof Palme, sources close to the investigation say.

LaRouche associates held a news conference Friday to denounce reports about the materials. They said the LaRouche organization had no contact after mid- 1985 with a man, Viktor Gunnarsson, initially held and released following Palme's Feb. 28 slaying.

NBC News reported that Swedish authorites are examining notebooks that contain 45 references to the Palme assassination, the use of a .357-caliber Magnum weapon in the slaying, and Gunnarsson.

''It is obvious that federal authorities are deliberately leaking false and misleading information to the press for the purpose of creating a prejudicial and inflamatory climate around their ongoing investigations of associates of Lyndon LaRouche,'' said a statement issued by LaRouche's National Democratic Policy Committee.

The LaRouche associates distributed an edition of his Executive Intelligence Review magazine devoted to the Palme slaying. Webster Tarpley, a LaRouche associate and contributing editor of the magazine, said ''the Soviets are the prime suspects'' in the slaying.

Sources close to a federal investigation into the LaRouche organization, who spoke on condition they not be identified, said there were numerous references to the Palme case in notebooks seized under federal search warrants on Oct. 6, when more than 400 state and federal agents raided LaRouche offices in Leesburg, Va. Sources said they appeared to have been written after the attack on Palme, but that they did not know what to make of the references.

''We're not going to comment on any ridiculous stories about notebooks,'' Tarpley said. ''Who knows what's in notebooks.''

A grand jury in Boston indicted 10 LaRouche followers and five of his organizations on charges of fraud and conspiracy to obstruct justice. A second grand jury, in Alexandria, Va., also is investigating the LaRouche organization.

Gunnarsson was publicly linked to the LaRouche organization after he was arrested May 12. He was released after several days but police picked him up several times to be shown to witnesses.

Irwin Suall, director of fact-finding for the Anti-Defamation League of B'nai B'rith, which monitors extremist groups, described Gunnarsson as ''not a hard core member'' but a ''sympathizer'' of LaRouche's Swedish organization.

Suall said he visited Stockholm last summer to look into LaRouche's operations there and was questioned by Swedish investigators. He said he was later questioned further by a Swedish consul official.

Suall said he gave the FBI a written report on his findings but added, ''Nothing in that report shed any light on the assassination.''

Palme had been the object of vicious attacks in LaRouche publications in the United States and even more so in Sweden, he said. Suall said LaRouche's Swedish publications referred to the prime minister as ''Ayatollah Palme'' and said he should be shot.

An American who heads the European Labor Party, which espouses many of LaRouche's ideas, disappeared after Palme's death, Suall said. He said Swedish authorities asked his help in locating Clifford Gaddy and his wife Kerstin Tegin Gaddy in the United States, but that he was unable to do so.

He said Swedish authorities said Gunnarsson and the LaRouche connection were among several leads they were following.