The swim-in scheduled to take place in the Potomac River on Labor Day was canceled yesterday under pressure from District harbor police, who said the event would be a "safety hazard".

Paul W. Eastman, director of the Potomac's watchdog agency, canceled the swim-in he had planned with some of his colleagues at the request of Lt. Thomas M. McGlynn, commanding officer of the harbor branch of D.C. police.

"It sure has been canceled," said Lt. McGlynn, who was irritated that "no one had permission to start with. No one contacted District officials. Swimming in the Potomac in Washington is against the law."

"It's a safety hazard." McGlynn said, "If you bring a lot of people in the water, you'll have a lot of drownings. The current is very strong, and the heavy flow of the water increases the speed of the current."

He also said that although the quality of the river is improving, it still contains too much bacteria from human and animal wastes for safe swimming - a contention that Eastman said is not borne out by recent samplings. Before police divers enter the river, one harbor policeman said, they are innoculated against possible typhoid infection.

Eastman and his colleagues at the Interstate Commission on the Potomac River Basin had planned the swim-in to dramatize their belief that the Potomac has been cleaned up so successfully that it is safe enough for bathing and other water-contact activites, such as skiing. Because of pollution, swimming in the Potomac has long been banned, and a 1971 District law punishes violations with a $300 fine.

Though he backed down, Eastman maintained yesterday that the Potomac is safe for swimming.

"I still think that if arrangement could be made for proper supervision, there are times when it would be perfectly safe. The implication of the law is that the river is still grossly polluted and that's not so."

He said currents are "considerably reduced in the tidal areas, except during periods of heavy flow." With proper supervision and more patrols - the D.C. harbor police at present have only four men on two boats - swimming would not be hazardous," he said.

Eastman said he hoped the swim-in would galvanize D.C. officials into thinking about opening the Potomac to water-contact activities. The raft race held last Saturday attracted 1,200 participants, he said, and was a clear indication of the recreational potential of the river.

But, he said, "It's a question whether anything will be done. It's a question whether I handled the situation in a way to make officials dig in at their heels."

A swimmable Potomac by 1975 was promised by President Lyndon B. Johnson in 1965. More than a billion dollars, most of it federal clean-water money, has been spent since then to upgrade sewage treatment plants that empty effluent into the Potomac. In addition, a number of raw-sewage outfalls have been closed on the Washington side.

Eastman maintained that the cleanup expenditures are paying dividends, and that it's time for the public to enjoy them.

Many environmentalists, though, have argued that even if there has been an improvement in the fecal coliform count - the primary indicator of water quality for human contact - the river is still polluted with other substances, including toxic metals, most of them the result of uncontrolled upstream runoff.

District officials say neither Maryland nor Virginia has a statewide policy controlling runoff. They also say that a new sewage plant in Montgomery County will send more pollution into the Potomac via Rock Creek.