BOSTON — Deena Kastor won the U.S. Olympic marathon trials today, earning a berth for the Beijing Games after erasing a gap of almost two minutes to pass runaway leader Magdalena Lewy Boulet in the last three miles.

Kastor, a bronze medalist at the 2004 Athens Games, had an unofficial 2 hours, 29 minutes, 35 seconds on the day before the Boston Marathon. She was joined on the U.S. women's marathon team by Lewy Boulet and Blake Russell.

Kastor, Lewy Boulet and Russell were all wrapped in American flags. They were handed U.S. Olympic team jackets and crowned with the olive wreath traditionally given to the Boston Marathon winner.

Lewy Boulet, who ran alone in front of the slow-paced pack for more than 20 miles, finished 44 seconds back. Russell missed a spot in the Athens Games by 35 seconds.

On a perfect morning for a marathon, with clear skies and temperatures in the high 50s, 146 women set out on a loop course that crossed back and forth over the Charles River before leading them back onto Boylston Street for the traditional Boston finish.

The roads and bridges were lined with thousands of fans, many of them runners planning to head to Hopkinton for Monday's 122th edition of the world's longest-running long run.

Wearing the favorite's bib No. 1, Kastor bobbed along in a blue singlet and white cap, taking off her sunglasses before making the final turn. She grabbed an American flag and waved it as she ran toward Copley Square, where no U.S. runner has claimed victory in the Boston Marathon since Greg Meyer won in 1983.

Kastor was also the favorite in the 2004 trials, but she struggled with stamina and strategy and earned her trip to Athens with a second-place finish. Her bronze medal was the first U.S. medal in the event since Joan Benoit Samuelson won the inaugural women's marathon in 1984.

On Sunday, the 50-year-old Samuelson ran what she said would be her last competitive race. She finished wearing a Red Sox cap as she did when she won Boston in 1979 for the first of her two titles. Her time of 2:49:08 was an event record for her age group.

Kastor was an overwhelming favorite with a personal best that was 12 minutes faster than the next-closest competitor. She sat back in a pack that watched Lewy Boulet move far in front. By 14 miles, she had a 1:56 lead over Kastor and about a dozen others. The lead at one point measured the length of the 2,000-foot Harvard Bridge, the longest of the spans across the Charles.

Kastor made her move with about 10 miles to go, and Russell followed as the pack began to string itself out. Lewy Boulet's lead was 77 seconds as they passed the finish line to begin their final, six-mile loop, but it was down to 30 seconds at 22 miles and eight seconds at the 23-mile mark.

As the runners passed in front of the stately columns of M.I.T., Kastor passed Lewy Boulet and built her lead.

A Pole who became a U.S. citizen on Sept. 11, 2001, Lewy Boulet finished fifth in the 2004 trials with a personal best time of 2:30:50, missing a chance to run in Athens by 53 seconds. She won in San Francisco and Pittsburgh in 2002 and, after taking time off to have a baby, she won the Orange County Marathon in 2006 and was 15th in New York.

At the 2004 trials, Russell took the early lead before wilting in the St. Louis heat and finishing fourth, the first alternate for Athens.