Sen. Richard G. Lugar, head of a team appointed by President Reagan to observe the Philippine presidential election said this morning that the group had found "disturbing reports" of electoral abuse and would report to Reagan about them on Tuesday.

"The word 'clean,' in its absolute form, is obviously not appropriate to what we have observed," Lugar said at a press conference here before leaving for the airport and home. Speaking of the tabulation of returns, Lugar said, "It appears that there were blockages, delays in the system, that manipulation of these results is a very good possibility."

He praised as "courageous" members of a citizens' pollwatcher group known as Namfrel that President Ferdinand Marcos has accused of manipulating returns to favor Corazon Aquino, his opponent in the election.

Lugar said Namfrel should continue its unofficial count of the ballots, which, in contrast to government counts, is showing Aquino in the lead. He also said that ways should be found to reconcile the conflicting counts. Officials in Marcos' government have said that the Namfrel count and other unofficial ones should be suspended today.

Lugar said the group was seriously concerned about statements by some of the 30 persons who walked off jobs at the government's vote-counting center last night that returns were being falsified there.

Lugar declined to say which side was committing most of the fraud, but he said that the government holds the ultimate responsibility due to the enormous authority it exercises. He said the group had received some reports of abuses by the opposition and that these should be investigated.

In tone and specifics, Lugar's statement was significantly weaker than that of an international team of observers that reported last night that they had found numerous cases of fraud by supporters of President Marcos.

"We saw many instances of vote-buying, attempts at intimidation, snatching of ballot boxes and tampered election returns," the international group said in a statement released last night.

Group coleader John Hume, a member of the British Parliament from Northern Ireland, said most of the abuses were carried out by "local officials who were supporters of the government." He said his group had found no evidence of cheating by the opposition.

Yesterday, amid reports that the Philippine National Assembly will consider calling a halt to the counting of unofficial returns, Lugar said on ABC-TV from Manila, "I would plead with President Marcos . . . to let the count continue." He said Marcos "has the power . . . to do about what he wants to do in this country," while Aquino does not have the power to organize widespread fraud.

"I would hope that reform might still come in this vote-counting process," Lugar said. "The political will to make the laws work, however, is at issue." He agreed that there had already been "massive fraud" but said, "There still could be a valid result . . . . The election is still going on. It's teetering on the brink of disaster, but it is still in place."

The observers' statements were another blow to the credibility of Marcos and his 20-year-old government, which has been under heavy pressure from domestic and foreign sources to undertake reforms.

U.S. officials have made it clear that a credible election, no matter who wins, is crucial if the next Philippine government is to have public backing for painful economic and military reforms and for combating a growing Communist insurgency.

President Reagan also made it a condition for increased U.S. aid; that aid totals about $260 million and is considered a weak lever at best. Marcos has said repeatedly that he would be saddened but not devastated by its withdrawal.

U.S. policy-makers have been meeting round the clock at the State Department to monitor reports from the Philippines. At stake in their desire to see a peaceful outcome are two huge U.S. military bases that are the chief U.S. foothold in Asia.

Concerned that Marcos might charge them with meddling in Philippine affairs because of earlier reports of U.S. "disgust" over fraud, the officials clamped a tight lid on unofficial comments.

Marcos has said that the election was one of the fairest the country has had. He has rejected charges of systematic fraud by his supporters and blamed what fraud there has been on the opposition.

The international observer group said that election irregularities occur to some extent in all democratic countries and that delegations members had visited areas in the Philippines where "balloting and counting have gone smoothly and without incident."

The group, which consists of 44 persons from 19 countries and was organized by the U.S. Republican and Democratic parties, said that it could not determine whether the abuses it found were serious enough to change the winner in the vote, which pitted Marcos against Corazon Aquino.

"Of great concern are reports that thousands of voters -- particularly in the Manila area -- were not able to vote because their names were not on the registration list," the statement said.

The delegation also criticized delays in reporting returns. "Forty-eight hours after the vote, we note that there are a number of competing counts, which are damaging to the credibility of the process," it said.

The team's statement said, "Our observations have been that the vast majority of Namfrel volunteers in the regions we have covered have acted in a nonpartisan, professional manner. Many have risked a great deal to preserve the integrity of the electoral process."

It said that in some areas Namfrel "was excluded from polling stations in violation of a range of previous agreements . . . . There were also some limited areas in which the system of safeguards seemed to break down completely, and those who held responsibility for adjudicating complaints were either unavailable, or unresponsive."

Delegation members from both groups were due to leave Manila today, however, they left some staff behind to continue observations.