Federal agents here have detected what appears to be a growing volume of illegal U.S. firearms shipments to the politically troubled Philippines and have arrested several Filipinos, including some with close ties to the government of President Ferdinand Marcos.

Although the suspected smugglers usually have said they are gun collectors with no political intent, federal officials and members of the anti-Marcos movement here say they suspect that anti-Marcos guerrillas and pro-Marcos private armies are buying the weapons.

Among five illegal shipments intercepted in the last eight months, investigators have seized Uzi submachine guns, assault rifles, shotguns, pistols, military-style laser scopes, submachine gun parts and conversion kits to turn MAC11 semiautomatic weapons into fully automatic machine guns.

Nolan Douglas, special agent in charge of the Treasury Department's Bureau of Alcohol, Tobacco and Firearms office here, said federal efforts to stop the traffic are proceeding nationwide, but much of the smuggling activity is centered in the San Francisco area. This city not only is a principal airline and shipping hub for trips to the Philippines, but also has a large community of Filipino immigrants with the right to buy machine guns on the open market.

Seizures "seem to have picked up quite a bit recently," said Thomas McDermott, assistant special agent in charge for the U.S. Customs Service here. The suspected smugglers say the intended recipient "is always . . . a progovernment official," McDermott said. "But if somebody was supplying the anti-Marcos forces, you would not expect them to disclose that."

A State Department official said concern over gun smuggling "has been a factor in our bilateral relations." He said Philippine government officials occasionally have made accusations -- often in response to U.S. criticisms of Philippine human rights violations -- that U.S. officials fail to enforce export laws.

Steve E. Psinakis, a San Francisco-based author and journalist who is a director of the anti-Marcos Ninoy Aquino Movement, said some of the weapons apparently are bound for non-Marxist opponents of the Manila government who have friends and relatives in this country. The Marxist New People's Army (NPA), now fighting a rural insurgency against Marcos, appears to steal or capture its weapons from government troops or buy them from corrupt officials.

Many of the arms, Psinakis said, are going to wealthy Filipinos who want to protect their own interests and have armed clout in the nation's faction-ridden, often violent politics.

The most prominent Filipino arrested this year for illegal firearms export is Douglas Lu Ym, acknowledged by his attorney to be one of the most important men in the Philippine coconut industry. He was handcuffed and taken off a Philippine Air Lines flight here Oct. 1 after customs officers found a Belgian-made assault rifle, a laser scope and a dismantled handgun in his luggage.

Lu Ym, Psinakis said, is a close associate of Eduardo (Danding) Cojuangco, a billionaire who controls a sizable portion of the coconut industry and is a prime backer and possible successor to Marcos. Cojuangco intrigues students of politically charged family ties in the Philippines because he is a first cousin of Corazon (Cory) Aquino, the widow of Marcos' assassinated rival, Sen. Benigno (Ninoy) Aquino. The cousins have been political enemies for some time, but there have been occasional reports of relatives trying to bring them together.

Lu Ym, now free on $50,000 bail but barred from leaving California, was apprehended after a bomb threat led to a special X-ray check of luggage aboard the plane. Roger Ruffin, his attorney, said Lu Ym is a gun collector who did not realize that his arms purchases could not be taken home.

In March, federal agents found two shipments of Uzi submachine guns en route to the Philippines, one of them addressed to Juan Ponce Enrile Jr., son of Marcos' defense minister. A Filipino identified by authorities here as Howard Mijares, 30, was fined $15,000, sentenced to two years in prison and deported after he told a federal judge that the weapons were for his gun club, the Philippine Practical Shooting Association. He said the club had been formed by his friends and university classmates, which included the sons of members of the Marcos government, including Enrile.

Ambassador Romeo Arguelles, the Philippine consul general here, said that although firearms smuggling is a serious crime, "some of the recent cases have been caught with just two or three pieces, not for terrorist purposes. Some people are fond of guns, and they just want to collect them."

Last Saturday, Douglas said, his agents arrested two Filipinos, Arturo and Carlos Colmenares, in a San Jose motel as the Filipinos tried to purchase 14 pistols. A shipment of recently arrived weapons found at the Manila airport led agents to the two suspects, Douglas said. A search of their San Mateo home uncovered five 12-gauge shotguns and parts for several MAC10 rifles.