Ten Americans, including former U.S. attorney general Ramsey Clark , defying a presidential ban and direct warning from the Justice Department, flew to Iran over the weekend to attend a three-day international conference on Iranian grievances against the United States.

The delegation of lawyers, churchmen and human rights activists had delayed departure from New Ork's Kennedy Airport Friday night after the Justice Department warned them that failure to abide by ban on travel to Iran ordered by President Carter last April 17 could result in individual penalties of up to 10 years in jail and fines of up to $50,000.

But Saturday night the Americans boarded a plane for London, where they contiued their flight. Wire services reported the group arrived in Tehran early today.

A larger U.S. group, up to 15 persons, according to some sources, had been scheduled to make the trip. But some decided to stay home, apparently because of the government warnings.

One of those was Princeton Prof. Richard Falk, who said he would not discuss his decision to stay or the others' decisions to go until the group's arrival in Tehran had been confirmed.

"I am reluctant at this point to talk about the trip, because of what the government might do," Falk said. "I decided, for various reasons which I cannont now go into, not to go."

The american Friends Committee in Philadelphia, through which the delegation issued a news release announcing its departure, said the group's travel schedule should have put it in Tehran by yesterday afternoon, but no word had been received.

A spokesman said the State Department had received no confirmation that the delegation was in Iran. The department said Saturday that the group's presence at the conference would be used for propaganda purposes and be "misconstrued" as U.S. acceptance of the conference's claim that the United States has committed aggression against Iran.

A spokesman for the Justice Department repeated the warning of penalties yesterday, saying, "If, in fact, they are over there, they know what they are liable for."

But he said he could not state at the moment whether action would be taken against the delegation.

The delegation said in a news release that it hoped its participation in the conference would "help to diffuse tension" between Ran and the United States.

The purpose of the conference, it said, was threefold:

To explain the nature of the Iranian revolution that brought Ayatollah Ruhollah Khomeini to power last year, and that subsequently led to the taking of American hostages in Iran.

To investigate "past (foreign) interference in Iranian affairs."

To "review the events surrounding the recent U.S. expedition in Iran" -- the failed U.S. military attempt April 24 to free the hostages.

The self-appointed American delegation said it has been assured by Iranian authorities "that the conference will entertain discussion for every relevant issue, including the great concern for the American hostages."

President Carter said in a television interview broadcast yesterday that that concern continues unabated. He also renewed his threat to take "very severe" retaliatory steps against the government of Iran if the American hostages there are put on trial "or are abused in any way."

Carter was not questioned about the American delegation to the Tehran converence in his appearance on "Face the Nation" (CBS, WDVM).

But he noted a statement issued by the United States shortly after the hostages were seized Nov. 4. "It still prevails," he said. "In effect, it prescribes what actions our nation whould preserve as options if the hostages are tried or abused in any way. Those actions would be very severe." h

Carter said it would be inappropriate for him "to spell in detail" what measures might be taken, should they be considered necessary by his administration. But he strongly implied, as he has in the past, that retaliatory steps might involve military force.

Besides Ramsey Clark, those making the journey to Iran are Kay Camp, international president of the Womens International League for Peace and Freedom; Nobel laureate scientist George Wald; the Rev. Paul Washington, an Episcopal minister and Philadelphia civil rights activist; Los Angeles lawyer Elonard Weinglass; the Rev. John Walsh, a protestant chaplain at Princeton University; the Rev. Charles Kimball, a Baptist minister who traveled to Iran last Christmas with Walsh to visit American hostages; Leonx Hinnds, a law professor at Rutgers University; free-lance journalist John Derrase, and Mary Anderson of the American Friends Service Committee.