SEOUL, South Korea (AP) _ The South Koreans, world-class friskers and scanners, are drawing their tight security net even tighter around the Olympics as the sports spectacular heads toward its final days.

Seoul's tough-talking student radicals are threatening to disrupt Sunday's marathon. A U.S. security specialist says his biggest worry is the anti- Americanism surfacing among ordinary South Koreans.

One Korean official finds the real security threat coming from the athletes themselves - one might defect and put the Olympic hosts in an awkward position.

After 12 days of Games, the security concerns at Seoul are often in the eye of the beholder. But by Wednesday everyone could see the new Olympics-wide clampdown.

For the first time, for example, police bomb-sniffing dogs were deployed at the Olympic weightlifting hall. At the Olympic press center, the security men blocked driveways with more steel barriers, checked car trunks and undercarriages for bombs, and closely searched visitors and journalists.

''We increased the vigil,'' one Olympic security official said.

The tightening up coincided with Tuesday's announcement that an anti- government student leader jailed since last week had been formally charged under the tough National Security Law, which covers sedition and similar acts.

Oh Young-sik's followers have threatened to disrupt Olympic events, including Sunday's men's marathon, a 26-mile run through the heart of Seoul, unless he is freed.

The marathon, vulnerable because of its length and route, is expected to be well-guarded by police strung out along the boulevards and by commandos trailing along in armored vehicles.

But some questioned whether Seoul's militant students were the main target of the stepped-up security, since the students favor symbolic street clashes with police, rather than car bombs or other terror blows against innocent civilians.

Had authorities detected a sudden threat from international terrorists, such as the shadowy, ultra-leftist Japanese Red Army?

''Categorically, no,'' said an American official who works closely with the South Korean security authorities.

This informant, who like most security officials here spoke on condition of anonymity, suggested that the Koreans reinforced their security apparatus simply because it had grown more lax since opening day, Sept. 17. ''They had let their socks down a little bit,'' he said.

Of the Korean protective net, involving more than 100,000 security personnel, the American said: ''It's working. And not only have there been no big plots, there have not even been attempted plots. ...

''We're more concerned now about the mood swings of the Korean people, the anti-Americanism.''

Seoul newspapers have been flooded with letters and calls from ordinary Koreans angry over the U.S. athletes' unruly, ''arrogant'' behavior during the opening ceremony, Americans' boisterous cheering at sports events, the ''biased'' reporting by U.S. news media of an Olympic boxing incident in which Korean officials attacked a referee, and the theft of an expensive bar ornament by two U.S. swimmers.

On Tuesday night, another American got in trouble. Runner Johnny Gray was arrested after he kicked a taxi door in a dispute with the driver. He was later released.

Meanwhile, down at the Olympic Village, where the athletes reside, village Director-General Ryu Keun-ha has other things on his mind.

He told The Korea Times that his security people would thoroughly check names of departing athletes against official lists to ensure that none tries to defect.

''We will pay our utmost attention to prevent any undesirable incidents,'' he was quoted as saying.

In an unusual move, the Seoul government, eager to establish relations with East bloc countries, has announced it will not grant political asylum to any athletes who seek to defect.