The east coast of the United States was bracing itself last night for one of the fiercest hurricanes for decades as 105mph winds made their way up the Atlantic, bringing large sea swells towards North Carolina.

Hurricane Isabel, a category two storm on the one to five Saffir-Simpson scale, is expected to strike as early as this evening with sufficient strength to tear roofs off houses and push tides between three and four metres (9ft-12ft) higher than normal.

"If Isabel stays close to our forecast track, and if it does make landfall as a major hurricane, it has the potential for large loss of life if we don't take it seriously and prepare," said Max Mayfield, the director of the national hurricane centre.

"I would expect to see extensive damage to a pretty large section of the country. It's been a long time since we've had a hurricane on this track."

The US homeland security secretary, Tom Ridge, said the hurricane could cause "significant damage and loss of life".

"We encourage families and communities who appear to be in the projected path of the hurricane to take the necessary precautions," he said on NBC's Today show.

The US hurricane centre posted a hurricane watch along much of the eastern coast, from South Carolina to Virginia.

Thousands of residents of small coastal islands and up to 75,000 people in the low-lying Dare county, North Carolina, were issued with evacuation orders.

The navy and air force moved planes, ships and submarines to prevent damage as the hurricane appeared likely to move north through Virginia, Washington DC, Maryland and Pennsylvania, possibly reaching New York and New Jersey.

Ed Rappaport, deputy director of the hurricane centre, said Isabel would "be one of the strongest storms seen in the landfall area in the last several decades".

Virginia has declared a state of emergency, putting national guardsmen, state police and transport crews on full alert, and Baltimore has cancelled leave in the police, fire and public works departments.

The hurricane centre's chief forecaster, Stacy Stewart, told USA Today: "Several hundred thousands of people, even millions of people, could be without power for several days. We expect this hurricane will be a major hurricane when it hits."

Residents of North Carolina's Outer Banks were ordered to leave on Monday night, including the 769 residents of Ocracoke Island - accessible only by boat - and hundreds more on Bald Head Island at Cape Fear.

Isabel has weakened significantly since Sunday, when it was a category five, but it remains stronger than the last big storm to hit North Carolina, Hurricane Floyd, a category two storm which killed 56 people and caused damage amounting to $4.5bn (about £2.8bn) in 1999.

"We aren't forecasting too much more weakening for the next 24 hours," said Krissy Williams, a meteorologist at the hurricane centre.

A colleague, Eric Blake, warned against being lulled into a false sense of security by the storm appearing to be waning, as it was likely to pick up again as it approached land.

"Hurricanes are notorious for gaining strength as they cross the gulf stream," he said. "Even [as] a category two hurricane, there's still a lot of potential for danger. People need to be prepared."

Residents along the projected path of the storm stocked up on torches, batteries, plywood and bottled water.

Last night Isabel was just over 500 miles off the North Carolina coast, heading north-north-west at 8mph. While North Carolina was steeling itself to bear the brunt, experts warned that two days in advance the forecasts could be wrong by 150 miles either way.

There was concern that the emergency services would be inadequate, with so many of the national guard engaged overseas. More than half North Carolina's national guard are either abroad or stationed elsewhere in America.

Meanwhile the Red Cross, which provides shelter and food, made an emergency plea for donations to cope with the hurricane, saying its disaster relief fund was nearly empty.

"If you're driving down the road and the gas gauge is bouncing off zero, then it's time to get some gas," said Alan McCurry, its chief operating officer.