Philippine President Ferdinand Marcos goes to Washington this week for his first U.S. state visit in 16 years in the wake of a crackdown that has weakened his domestic opponents but given more ammunition to his critics abroad.

While he has played down the visit as little more than a social call on President Reagan, it is clear that Marcos attaches much more importance to it as a major milestone of his 17 years in power. Preparations for the visit have been frenetic and advance teams have been working on it in the United States for four months.

The prime mover behind the visit is the new Philippine ambassador to Washington, Benjamin Romualdez, who is also the president's brother-in-law and governor of a southern province.

When Marcos arrives in Washington for talks Thursday with Reagan, officials here say, his entourage will include at least a dozen Cabinet ministers and several hundred other persons serving in some capacity, private or official.

Among the subjects Marcos plans to discuss with Reagan are the role of the United States in the Pacific region's security, new agreements governing two major American military bases here and economic and trade matters. But at least as important as these to Marcos will be the image he wants to project back home as leader of a former American colony meeting the American president in Washington on equal terms.

"It's important for him to show that he's a close ally of the United States, but that he's going there as an equal, not like in the past," a Western diplomat said. He recalled Marcos' last state visit to the United States in 1966, when, nervous and inexperienced in his first year in office, he went more or less hat in hand "looking for money and support" from a somewhat patronizing then-president Johnson.

Marcos has been in the United States since then but only on private and ceremonial visits.

In this perspective, the visit goes to the core of the legacy Marcos is trying to build for himself, contributing to the aura of a leadership that has already taken on some of the characteristics of a monarchy.

But while Marcos will be meeting a president considered here to be the best friend he has ever had in the White House, the visit also is fraught with dangers.

Marcos' supporters worry that opponents here and in the United States will embarrass and discredit him with massive demonstrations.

Especially irksome were plans by opposition labor leaders to stage strikes and protests this month, while the Philippine economy is suffering from sharp drops in world commodity prices affecting its leading exports.

In response, Marcos last month ordered the arrests of three leftist labor leaders and scores of antigovernment agitators. They were accused of involvement in a communist plot to carry out strikes, bombings, assassinations and the armed overthrow of the government by 1983.

In a news conference last Monday, Marcos said investigators had found handwritten evidence of such plans and had captured hundreds of firearms and explosives. He said the government's "preemptive action" had thwarted the alleged plot. Nevertheless, security forces have been alerted to guard against disturbances, especially while the president is away on the trip, which is expected to last 10 days.

Saturday, a bomb exploded in front of an overseas employment agency on a busy downtown street, killing three persons and injuring more than 20, police said. But the motive was not immediately clear.

According to Western diplomats and other independent observers, the crackdown so far generally seems to have worked. In addition to the arrests of the labor leaders and antigovernment agitators -- 42 are now detained and 26 others are being sought -- Marcos at the same time created a 1,000-strong force of "secret marshals" empowered to kill suspected common criminals. At least 47 suspects were killed in two weeks last month before the marshals' powers were restricted.

In some ways the tough policies and charges of a plot against the government reminded Filipinos of the days before Marcos declared martial law in 1972, a year ahead of his scheduled departure from office at the end of his second presidential term. Marcos ruled under martial law until January 1981, when he lifted it and secured constitutional changes allowing him to run for a new, six-year term. He won reelection handily in June 1981 in a contest boycotted by most opposition groups.

"Now Marcos is giving a signal to people in the Philippines that he is still in control," a Western diplomat said. Marcos also could be signaling Washington that there is a potential threat to him, he added.

A Marcos aide agreed that the crackdown "served as a reminder to everyone concerned of who is in charge, whether he [Marcos] is in the country or abroad."

Diplomats and other observers said the action appeared to have taken the wind out of an increasingly militant part of the labor movement and had discouraged moderate opponents, who scheduled but then called off several protests.

"I think the real purpose of Mr. Marcos was just to generate a climate of fear so that in his absence people would be too scared to do anything," said former senator Salvador Laurel, the head of a moderate opposition grouping. However, he noted that Marcos opponents among the estimated 750,000 Filipinos living in the United States were using the crackdown as a rallying cry for demonstrations against the state visit.

Laurel said a major test for the moderate opposition would come Tuesday, when a "massive demonstration" is planned in front of the Manila cathedral. He said organizers hope to attract 25,000 to 50,000 people to protest the state visit. The only demonstration against the trip so far has been the gathering of about 70 students in front of the U.S. Embassy here last week.

Besides the domestic opposition, Philippine authorities also are concerned about the activities of opponents in the United States led by former senator Benigno Aquino, who was jailed under martial law for eight years before Marcos freed him to undergo heart treatment in Texas. Aquino later took up a fellowship at Harvard.

Armed forces Chief of Staff Fabian Ver accused Aquino Friday of collaborating with Philippine "criminals" in organizing anti-Marcos demonstrations.

Laurel said that besides their aim to end U.S. support for the Marcos government, opponents here also opposed the state visit because "it's not worth all that expense at a time when we're broke." He claimed it would cost "$20 million at least." Government officials dismissed criticism of the visit's cost, which they refused to estimate.

A Western diplomat said, however, "Whatever the trip costs, they'll get it back in compensation for the bases."

He was referring to what is expected to be a main subject for discussion during the visit: the review of a 1979 agreement on the use of huge American naval and air bases here until 1991.

The talks are not expected to go into great detail, but Marcos has indicated that he wants agreement soon on a date to begin "renegotiating" the accord before its five-year review in 1984. Officials here have said they would like to start the talks by January 1983.

The Philippine side also is expected to raise the issue of U.S. compensation for use of the bases. The current arrangement provides for a $500 million U.S. aid package during the five years, a figure that Philippine authorities consider far too low. No revised figure has been proposed officially, but the numbers mentioned informally have ranged as high as $2 billion, informed sources said.

The higher "rent" -- as the Filipinos prefer to call the compensation -- undoubtedly would bolster the economy, which is sagging under a $16 billion foreign debt and low prices for the principal exports of coconut oil, copper, sugar and lumber.

The poor economic conditions in depressed rural areas have fueled the slow but steady growth of the communist New People's Army, which has been conducting an insurgency against the Marcos government.

While the bases agreement is considered the most important issue in Philippines-American relations, Marcos has indicated that he is most interested in assessing during his U.S. trip how the United States envisions its future role in Asia and its commitment to the security of the Philippines.