A massive ice floe, estimated to be about 15 miles long traveled down the swollen Potomac River to Washington yesterday, causing a major mobilization of police and emergency crews but little damage by nightfall.

The National Weather Service said the floe was made up of large chunks of ice, many of them about one-foot thick, that moved downriver together with a flood crest, caused by hard rains earlier in this week.

The Weather Service warned that if the floe became jammed around Washingon's bridges, the ice could come ashore and cause serious damage.

The U. S. Army Corps of Engineers used cranes with wrecking balls to break up the ice underneath Chain Bridge so that it would keep moving down river. Other cranes were readied for use on Key Bridge, Theodore, Roosevelt Bridge and the Arlington Memorial Bridge.

Broadcast stories about the ice threat brought thousands of sightseers to look at the Potomac. Many parked their cars along both the Virginia and Maryland sides of the George Washington Memorial Parkway.

Police closed off the overlooks on the Parkway and during the afternoon closed parts of the roads and bridges also.

"The problem is spectators," said George Rodericks, the director of the city's Office of Emergency Preparedness in the late afternoon. "It's a nice Saturday and it's sunny. There's no football so everybody thinks they might as well go down and watch the ice floe. But we have to make sure crews can get in if something serious happens."

Leo Harrison, river forecaster for the weather service, said the ice floe started Wednesday on the south branch of Potomac River near Springfield, W. Va. He said hard rains caused the river to rise rapidly, lifting and cracking its thick cover of ice. The ice moved downstream in a long train, Harrison said, carried by the flood-waters.

It reached Hancock, Md., about 4 p.m. Friday, causing one or two inches of water to cover downtown streets.

There also was minor flooding at Harper's Ferry, where the ice floe passed at about 6 a.m. yesterday.

The ice floe and flood waters reached Great Falls, Va. in mid-morning and the Cabin John Bridge, which carries the Capital Beltway, about noon.

The slabs moving down river broke into smaller chunks as they passed over Great Falls.

However, the floe virtually stopped where the Potomac narrows just south of the Cabin John Bridge.

Smaller chunks of ice continued to float down river, reaching Georgetown and Roosevelt Island in mid-afternoon. The island was evacuated at about 3 p.m.

"We are trying to keep people away, that's why it boils down to," said Richard Shafer of the U. S. Park Police. "The water is three feet from Virginia bank of the river and all the way to the D.C. bank. It has the power to move things and we don't want anyone hurt. So in case the water breaks we've been evacuating all areas around there."

Police closed temporarily both Key and Chain Bridges. Traffic was forbidden on Chain Bridge as a protective measure, police indicated. Key Bridge was shut down by police because of traffic jams caused by Saturday shoppers and sightseers drawn to the bridge to view the ice floe. The tangle of cars prevented tow trucks from reaching cars damaged in an accident during the traffic jam.

The Weather Service said long ice floes had been exprienced in the Potomac only twice in the past 100 years, mostly recently in 1933. The ice jammed then to a depth of 15 to 25 feet, causing serious damage to the shore.

Harrison said the area of greatest potential danger from an ice jam was in Washingon between Memorial Bridge and the Georgetown water front. He said that if ice does jam there, it migh cause flooding of up to three feet.

A major jam was possible there, he said, because water begins moving more slowly in that area as the river widens and flattens out.