Flooding caused by continued heavy rains along the Atlantic Coast has left at least 26 persons dead in Virginia, Maryland and West Virginia, and experts fear that the death toll may rise as rivers swollen by as much as 20 inches of rain in the past few days crest at near-record levels.

Most of the deaths occurred as swiftly rising water swept through the Roanoke-Lynchburg area, washing out roads, bridges and power lines and trapping some victims in their houses and cars.

Virginia Gov. Charles S. Robb and West Virginia Gov. Arch Moore declared emergencies in parts of their states as property damage estimates passed the quarter-billion-dollar mark. Moore asked President Reagan to declare portions of West Virginia a federal disaster area.

The Potomac River was cresting early today in Paw Paw, W.Va. -- about 35 miles west of Harpers Ferry -- at 54 feet, nearly 29 feet above flood stage. It washed away a temporary bridge that spans the river between Maryland and West Virginia.

Forecasters at the National Weather Service said the Potomac should crest at least 12 feet above flood level at Harpers Ferry about 10 p.m. today and reach its crest here about 2 p.m. tomorrow, rising about six feet above flood stage, or 12 1/2 to 13 1/2 feet.

In 1972, after torrential rains caused by tropical storm Agnes, the river reached 16.7 feet in Georgetown and wrought one of the worst flooding disasters in Washington history.

The flood levels predicted for tomorrow could send water over the sea wall and cover the roadway along Rock Creek Parkway near the Kennedy Center. Flooding also is expected at the Jefferson Memorial, along the the George Washington Memorial Parkway, and in parts of Old Town Alexandria, officials said.

Traffic officials said last night they did not know how the rising water would affect commuters tomorrow. The flooding caused by Agnes created monumental traffic problems throughout the area.

Similar near-record levels are expected for the James River, which forecasters say should crest in Richmond about midday tomorrow at 25 feet; officials there said the warehouse district could be damaged. The mark set on the James after Agnes was 28 feet.

Officials last night expressed concern about tanks of chlorine and propane that had been swept away from industrial sites near Lynchburg and into the James River. "With the rate of the river, [the propane tanks] should be like torpedoes if they hit some of these pilings," Lynchburg Fire Chief William A. Anderson said.

Earl Kittleman, spokesman for the National Park Service, said, "We anticipate the damage from the flooding to be as great as Hurricane Agnes in 1972." The entire 184-mile C&O Canal towpath was closed yesterday for the first time, and sandbagging operations were begun around Great Falls Tavern.

Authorities listed 18 dead in Virginia, seven in West Virginia and one in Maryland, plus one in Pennsylvania, but many persons caught in the storm were missing.

In the Washington area, more than 100 Maryland residents in the low-lying areas of Anne Arundel, St. Mary's and Baltimore counties were forced to evacuate their homes. Maryland's St. George Island was under three feet of water in the Potomac.

Authorities issued a flood watch along the length of the Potomac, and streets in Old Town Alexandria were barricaded.

Some merchants sandbagged their storefronts in Alexandria, where flooding yesterday had turned the intersection of King and South Union streets into a canal.

"It was a disaster," jeweler Ramesh Gidwani said of the damage to his store. "I heard about the flooding, but I didn't think it was going to be so high. I sold my house to open this business, and now this happens right before Christmas. This has left me helpless."

In Salem, Va., 239 residents spent Monday night at the local civic center after abandoning their homes, and some were still camped out there last night eating dinners provided by the Red Cross.

"I was at work. I don't even know what happened. All I know is that I can't go home," said Angela Brown, 31, who sells clothing for a manufacturer there. "I know that there's enough damage so that all our stuff is probably gone. We're hoping we can get this month's rent back so we can find another apartment."

Eldridge Massey, 47, who was spending the night at the civic center with his mother, said they were rescued from their flooded house by Roanoke firefighters in a canoe.

"The water came up through our furnace floor," Massey said. "I imagine we lost everything. Everything's totally gone, and we don't have any insurance at all."

"From what I've heard, it appears as if this is going to be as bad if not worse than Hurricane Camille" in 1969, said Michael La Civita, spokesman for the Virginia Department of Emergency Services. "It's a serious flood. It's not to be taken lightly. We're looking upon it as a very serious flooding situation."

The National Weather Service predicted that the rain would ease in the Washington area, diminishing to possible showers and partly sunny skies today.

The continuing downpours left flooding over a broad swath of the Atlantic Coast region as far west as Ohio and including parts of Pennsylvania and New York.

On Sunday the storm swept across Florida from the Gulf of Mexico, and it reached its greatest intensity as it gathered strength over the Carolinas. The storm whipped up gale-force winds along the coast and left many areas awash in torrents of rain.

Some areas reported as much as 20 inches of rain in the past three days, although most of the Washington area was spared the brunt of the storm.

The hardest-hit communities were in western and southern Virginia and in West Virginia, where swollen streams and rivers broke barges from their moorings, swept away automobiles, trailer homes, bridges and livestock, and left entire towns cut off except for shortwave radio communications.

"It's like a 50-year flood," said Jim Graves, owner of the Graves Mountain Lodge in Syria, Va., where more than 30 vacationers were trapped and most telephones were still not operating yesterday. "Some of the cattle washed away. A lot of the bridges are washed out. People are walking five miles to the general store to get provisions."

In Marlinton, W.Va., between 700 and 800 residents were trapped on rooftops and spent most of the day yesterday waiting for rescuers to arrive, according to John L. Price, press secretary to Gov. Moore. "We have towns that are virtually underwater," Price said. "We have thousands on thousands of people that have had to flee their homes. We have millions of dollars in property damage."

Law enforcement officials there reported looting in some cities and complained that they were unable to respond because of the flooding.

"Half of Marlinton is gone," said Pocahontas County sheriff's deputy Craig Doss. "Several people, including me, lost their homes. Water took mine right down the river."

Authorities described a "wall of water" crashing through Roanoke as the Roanoke River rose 13 feet above flood stage, leaving mattresses, lumber and other debris floating in waist-deep water in the city's streets.

At least 100 residents there had to be flown away from danger by rescue workers in helicopters.

A railway worker was killed Monday night when a mud slide near Cumberland, Md., washed away tracks and sent two locomotives and 17 coal cars plunging down a mountainside into the Savage River. Two persons who were trapped on a truck in a West Virginia stream were swept away by flood water while firefighters attempted to rescue them.

Two persons drowned in houses in northwest Roanoke, and another woman drowned there when she was swept into a creek.

A man was killed and a woman injured when their car hit a tree that had been washed onto a highway in Franklin County, Va.