U.S. Park Police apparently discounted early warnings by both the FBI and the State Department that opponents of the shah of Iran were likely to clash violently with the shah's supporters during his state visit here two weeks ago.

The small force of Park Police officers assigned to the large demonstrations for and against the shah on his arrival Nov. 15 were overwhelmed by stick-swinging anti-shah Iranian students who attacked the shah's supporters on the Ellipse behind the White House, injuring scores of people.

Following that violence, it has since been learned, the White House hastily instructed Attorney General Griffin Bell to have the Justice Department take over coordination of law enforcement activity for the rest of the two days of the shah's visit and accompanying demonstrations.

The FBI said yesterday it warned Park's Police officials in predemonstration planning sessions that militant Iranian students were planning an organized "action" against the shah's supporters.

State Department security officials said they also expressed concern and warned Park Police of the long-standing and intense hostility between the bitterly opposed Iranian factions.

Park Police officials said yesterday that they received little such information and that it was "speculative" and "unconfirmed."

The Ellipse melec, in which 96 demonstrators and 28 policemen were injured, prompted criticism; including private grumbling by some officials of the D.C. police department, that the "low profile" policy of the park police for the demonstrations was ill-advised.

Several rank-and-file Park Police officers also have been openly critical of the handing of the incident and threatened to sue the police herarchy for allegedly failing to provide them adequate protection.

Park Police officers involved in the Ellipse incident say they were told to leave their riot gear - helmets, face shields, riot sticks and gas masks - on nearby buses.

When the student attack occurred just as the 21-gun salute for the shah sounded, the undermanned police were overturn. Most were unable to get back to the buses for their riot gear. Some reinforcements arrived and with the help of police mounted on horses, the anti-shah demonstrators were finally separated from the others and routed to the eastern part of the Ellipse.

Somewhat later, Robert J. Lipshutz, counsel to President Carter, called Attorney General Bell, who was then in the Southern Newspaper Publishers Association. Lipshutz told Bell that the White House wanted the Justice Department to "coordinate" law enforcement activities" for the rest of the demonstration period, according to a Justice spokesman.

Bell then called his deputy, Peter Flaherty, in Washington, who immediately sent Justice Department monitors out on the street. Flaherty also met personally with Park Police D.C. police and Secret Service officials at the White House, where the demonstrators were concentrated. James B. Adams, deputy associate director of the FBI, accompanied Flaherty as both men were briefed on the situation by law enforcement officials.

The following day, both Park Police and D.C. police appeared in much larger and more conspicuous numbers, and the opposing groups of demonstrators were kept at a greater distance from each other.

There were few incidents the second day, as the groups disbanded and went home.

"The police worked out security details with our concurrence." Flanerty said in explainint the Justice Department's relationship to local law enforcement agencies. "We wanted to be in on my major decisions or changes in plans . . . We basically played a low-key role."

Two high ranking officials of the FBI said the FBI had met with Park Police and other local law enforcement officials before the demostrations and warned of possible clashes between the anti-and pro-shah demonstrators.

"All the indications were that the anti-shas (faction) was planning an action," said Jim Ingram, an assistant to Adams. Asked if the information was based on hard intelligence. Ingram added. "it was substantial information. It was verified and passed along (to Park Police),"

Paul L. Mack, executive assistant to Adams, said the information came from "various sources," but neither he nor Ingram would specify them.

Caron McConnon, a State Department spokeswoman, said state security officials also passed on information to Park Police indicating the "potential for a clash."

The information, developed by the department's Iran desk, was more of a "general historical sort" than hard intelligence, she said. Ann Iran desk official, who asked not to be named, said. "We updated (police agencies) on Iranian student activity . . . Our basic conclusion? We were going to have trouble . . . Our security people were very concerned."

He noted that there has been a volatile and long standing dispute between some Iranian student factions and shah supporters over the human and civil rights policies of the shah.

Park Police Chief Jerry L. Wells and other officials of both the Park Police and the Interior Department, contend they received little or no advance information from the FBI or State Department. "There was no indication given to me . . . that there would be any fighting or anything like that," Wells said yesterday.

"They did convey one item," he said." But that was unconfirmed. They said the Iranians were buying large quantities of oil in their communities (in the U.S.), but they didn't know why."

He said there was "some speculation that it could be used for some kind of molotov cocktail conrtaption . . . but that was just speculation."

No evidence of molotov cocktails was found during the demonstrations, but police did confiscate numerous clubs and shields used by the demonstrators.