BOSTON (REUTERS) - Joe Anderson had just crossed the finish line of Monday's Boston Marathon when he heard the blasts that killed two people, wounded dozens and terrified thousands of spectators and athletes.

The first blast, which blew out the window of a store near the end of the race course at around 2.50 pm local time (2.50am Tuesday Singapore time), stunned the throng of people who had gathered in Copley Square for the race's finale. A few seconds later, a second blast sent waves of fear and horror through the crowd.

"I saw multiple injuries. I saw people who looked like they had their legs blown off. There was a lot of blood over their legs," said Mr Anderson, a 33-year-old fisherman from Pembroke, Massachusetts, who had carried an American flag for the 42.16 km length of the race.

The flag was still draped over his shoulder later on Monday afternoon, as he slowly walked away from Massachusetts General Hospital, where many of the wounded were being treated.

"Fly that flag high," a bystander shouted.

For Americans, the blasts called to mind the attacks of Sept 11, 2001, which destroyed the World Trade Center in New York and damaged the Pentagon in Washington.

The blasts came about 2-1/2 hours after the winners crossed the finish line, but more than 5,700 amateur runners remained out on the course at the time.

"They stopped us half a mile away from the finish line," said Ms Christine Haverly, 20, a Boston College student from Erie, Pennsylvania. "They just said two bombs went off, but they didn't go into details about anything. We were standing there for like half an hour."

Runners crossing the line could be seen hitting the stop buttons on their watches just as the first explosion blew out the windows of a sporting-goods store along the course, spraying glass into the crowd.

"All the runners just stopped," said Ms Kathleen Earley, a 26-year-old teacher from Boston, who was about a mile from the finish line when the blast occurred.

"I was just hoping my friends and family were OK," she said.

Her mother, Rita, had come to the finish to cheer her daughter and emerged from a subway station just minutes before the first blast.

"Right away it seemed like something was really wrong, just looking at people's faces," the 61-year-old biologist said. "It was sort of reminiscent of 9/11 because everyone's on their cell phones, everybody wants information."

The marathon is run on Patriots' Day, a Massachusetts state holiday when some offices close, while students at the state's schools had just begun a week's vacation, helping to swell the crowds along the race route.

"We just ran for our lives," said Ms Laura Peterson, 18, who had gone to the finish to cheer her mother, Bridget, who she later reunited with. "I was just waiting for another bomb to come down and hit me."