James G. (Bo) Gritz, a former U.S. Special Forces officer sought for leading raids into Laos to search for American prisoners of war he believes are still being held there, today surrendered to police in northeastern Thailand.

Wearing a bright red shirt and dark pants, Gritz, 44, calmly walked into the district police headquarters in Nakhon Phanom, the town on the Mekong River from which he launched forays last November and, allegedly, last month into the jungles of neighboring Laos, a police spokesman said.

Gritz (whose name rhymes with lights) refused to say where he has been since plans for a new expedition were disclosed last month. But he told reporters at the police station that the results of his efforts have been positive.

Gritz also said he had just learned of the Feb. 13 arrest of two alleged associates, another former Green Beret and the daughter of an American pilot shot down over Laos, who were supposed to go on trial today on charges of possessing illegal radio equipment at a rented house in Nakhon Phanom. The trial was postponed.

The two Americans, Lance Edward Trimmer and Lynn Standerwick were jailed for two days, then released on bail to await trial on the charges, punishable by a five-year jail term. Also found in the house were fatigues, scuba diving equipment, jungle knives, cartridge belts and a gas mask.

Gritz told reporters he decided at 3 a.m. to give himself up to protect Trimmer and Standerwick, who he said were "completely innocent."

The former Green Beret lieutenant colonel, a decorated Vietnam War veteran, refused to discuss reports that he reentered Laos Jan. 30 with a team of former Special Forces comrades and anticommunist Laotians to pursue the search for American prisoners of war.

In an interview with the Los Angeles Times in Nakhon Phanom today, Gritz insisted, "One thing is certain. There are Americans alive and in captivity. Before, I was never certain. I'm certain now." He did not say where he believed the Americans were being held or how he obtained his information.

He said he would tell what he knows to U.S. officials and that it is up to them to obtain the prisoners' release.

Last Wednesday, Vietnamese Foreign Minister Nguyen Co Thach repeated his government's assertion that no Americans are being held in Vietnam, Laos or Cambodia. In an interview in Hanoi he called Gritz's actions "a flagrant violation of the sovereignty of Laos that everyone should denounce."

A western diplomat in Hanoi said, however, that he was "very sure" that some other American servicemen are living in Vietnam. He said they were deserters who had renounced U.S. nationality. A senior U.S. diplomat in Bangkok confirmed that the embassy here has the names of some deserters living in the Hanoi area, but he declined to say how many.

In a handwritten message purportedly sent by runner from the Laotian jungle and delivered to the Los Angeles Times bureau in Bangkok last week, Gritz claimed to have "some POW ID" that he was trying to "personally confirm."

In the letter dated Feb. 12, Gritz said that a second Laotian member of his team had been killed. In his November mission, one Laotian guerrilla was killed, three were wounded and an American radio man was captured when rival anticommunist resistance fighters ambushed Gritz's group, according to a participant, former Special Forces sergeant Charles J. Patterson.

The letter was signed by Gritz and two associates, former Navy airman David S. Weekly of Encinitas, Calif., and Gary Goldman, of Encino, Calif., whose whereabouts are still unknown.

Gritz told reporters the two other Americans would be following him, but by evening nobody else had shown up at the police station where Gritz was being held.

Appearing healthy, rested and clean-shaven, Gritz told reporters that he had come in from a "long walk" and wanted to assume responsibility for the radio equipment that Trimmer and Standerwick were charged with possessing. On his wrist he wore a bracelet bearing the name of Lynn Standerwick's father, Lt. Col. Robert Standerwick, who bailed out of his F4 Phantom jet in February 1971 while flying a reconnaissance mission over the Ho Chi Minh Trail in eastern Laos.

The Air Force pilot thus became one of nearly 2,500 American servicemen still listed as missing in the Vietnam War. Most, including Standerwick, are presumed dead.

In Nakhon Phanom, Gritz was questioned for several hours in a cordial atmosphere after he walked into the police headquarters with a Thai lawyer at 7 a.m.

"I'm Bo Gritz," he announced, to the astonishment of a few reporters and an American consular official who had shown up for the trial of Trimmer and Standerwick.

Gritz told reporters that publicity had jeopardized his mission. He lashed out at his former comrade, Patterson, who sold a story about Gritz's activities to the U.S. magazine for mercenaries, Soldier of Fortune, for $5,000.

Patterson has said that actor Clint Eastwood pledged $30,000--in exchange for book and movie rights--to finance Gritz's November mission into Laos code-named Operation Lazarus. William Shatner of the "Star Trek" television series also has said he gave Gritz $10,000 for an option on his life story.

Patterson has said President Reagan expressed his support for Gritz's November mission, which was intended to seek out and, if possible, rescue 120 Americans who Gritz believed were being held at three camps in the Xepon area of eastern Laos.

The White House has acknowledged that Eastwood informed the president of plans for the raid last fall, but denied that Reagan approved it and insisted that Gritz was advised to cancel it.

According to Patterson, the foray was launched Nov. 27 and the team was ambushed three days later. The radio man, Dominic Zappone, was captured and later ransomed, and the team lost all its equipment, including two high-speed code transmission devices, Patterson has said.