House Democrats returning from an inspection tour of Grenada endorsed President Reagan's invasion of the small Caribbean island yesterday, prompting House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) to declare that the military operation "was justified."

O'Neill had challenged the president on Grenada last week, saying, "Mr. President, your policy is wrong." Yesterday, he issued a statement supporting the invasion after meeting with a congressional delegation he sent to the island last weekend.

"The overwhelming consensus of the members of the delegation was that a real potential threat to the American citizens existed in Grenada," the speaker said last night. "Since this was the case, I believe that sending American forces into combat was justified under these particular circumstances."

Thus the Democratic leader in the House, an outspoken critic of the Republican president on most domestic issues, has now given Reagan an imprimatur on both of his two recent military endeavors--in Lebanon and Grenada.

O'Neill's statement, together with strong support for the invasion from most Democrats who made the fact-finding tour, should serve to spare Reagan any significant political criticism on the Grenada operation. Many Democrats, sensing the grass-roots support for Reagan on the issue, had already muted their comments. O'Neill had been one of the few Democratic leaders to take the issue on.

As a legislative matter, the positive report from the House delegation probably means that Reagan can win authorization from Congress if he wants it to keep U.S. soldiers on the island for several months. Congressional leaders say the U.S. occupation of Grenada is governed by the War Powers Resolution of 1973, a law that says U.S. troops sent into hostilities overseas must be withdrawn within 60 days--Dec. 24 in the case of Grenada--unless Congress authorizes a longer stay.

"I hope our troops will be off the island by the end of the year," said Rep. Bill Alexander (D-Ark.), the Democrats' fourth-ranking member, who made the trip to Grenada. "But if the president asks for another 30 days or 45 days or whatever, we can approve that."

House Majority Whip Thomas S. Foley (D-Wash.), who led the Grenada delegation, said that "a very large majority of our delegation believes the president acted correctly to protect American lives."

Foley and Alexander both remarked that they were impressed to find Grenadians approaching them in the streets to thank them for the American invasion.

Rep. Michael D. Barnes (D-Md.), a liberal who has criticized Reagan for using threats of military action where he said diplomacy would serve, said he concluded from his weekend visit that "in this limited instance in Grenada the use of force was justified" because "there was a sense of terror on the island, not only among American nationals but Grenadians as well."

The strongest factor in causing him to decide that the invasion was necessary was "the fact that our citizens were in danger or had good reason to believe they were in danger."

But Barnes, chairman of the House subcommittee on Latin America, said he did not think U.S. troops should remain on Grenada past Christmas.

Alexander was one of several Democrats on the trip who said the remarks of Americans and Grenadians on the island left him with no doubt "that on Oct. 25, when our troops went in, all American lives on that island were in imminent peril . . . . There was anarchy in the country. The only force was this army, a group of people ranging from 12 to 28 years of age, walking around with rifles. One person told me that they were 'a gang of bullies.' "

Alexander said he was also basically satisfied that the United States had attempted to safeguard Americans on the island through non-military means before deciding to invade.

He said he was told that the State Department had "pleaded" with the administration of St. George's University, the medical school with 600 American students, to close and send the American students home earlier this fall. He said the U.S. government had even offered a decommissioned naval base on nearby Antigua as an alternate campus, but the school turned down that proposal.

A group of four conservative Republican congressmen who made a separate Grenada tour--paid for by the National Defense Council, a private group based in Alexandria, Va.--returned with an accumulation of documents and military paraphernalia that proved, they said, deep Soviet involvement with the revolutionary movement the United States toppled when it invaded.

Rep. Don Ritter (R-Pa.) displayed a 1983 diary he found in the rubble of a Grenadian government building. There was no name on the diary, but Ritter concluded it must have belonged to a leader of the revolutionary movement. In the diary were references to terrorism and to cash payments from the Soviet Union, possibly to someone in the revolutionary movement.

Meanwhile, Secretary of Defense Caspar W. Weinberger said yesterday that the invasion of Grenada will "help convince our adversaries that it is not in their interests to threaten freedom."

At a Labor Department ceremony honoring the nation's veterans, Weinberger said the operation "was carried out superbly" and showed that "once again, fortunately for us and for the world, it is an honor to wear an American uniform."

The defense secretary also announced that Soviet, Cuban and North Korean weapons captured in Grenada will be displayed to the public at Andrews Air Force Base beginning Friday. "We're bringing this home so we can have the American people see . . . that there was a very large, major armed camp right there in our back yard," Weinberger said.

Administration officials also said yesterday that $6 million in U.S. military assistance funds would be used to sustain the police force in Grenada, made up of small units from several neighboring Caribbean islands.