Shortly before 3pm on Monday, Amy Eshleman was approaching the 26th mile of the Boston marathon. The atmosphere along the route was more like a party than a major sporting event, with thousands of spectators enjoying the spring weather on a public holiday, Patriots' Day.

Then came heard the first explosion, just quarter of a mile away from Eshleman. "I heard it, I saw smoke. I thought maybe it was fireworks. People kept running forward," she told the Guardian. "My eyes were on the finish line."

Ten seconds later, a second explosion brought home the terrifying reality. Plumes of white smoke filled the air and runners were knocked to the ground by the force of the blasts. "People just stopped. They started crying," Eshleman said.

At first, runners and spectators just froze, unsure of whether to move. Brighid Wall, 35, of Duxbury, said her husband threw their children to the ground, and lay on top of them. Another man lay on top of them and said: "Don't get up. Don't get up."

After a minute or so had gone by without another explosion, Wall and her family headed to a Starbucks, and out the back door through an alley. Around them, the windows off the bars and restaurants were blown out.

Wall described six to eight people bleeding profusely, including one man who was kneeling, dazed, with blood rushing down his head. Another person was on the ground, covered in blood and not moving. "My ears are zinging," Wall told AP. "Their ears are zinging. It was so forceful. It knocked us to the ground."

Lisa Hickey was waiting for her daughter, Allie, 21, to reach the finishing line with another daughter, Shannon. "The first bomb went off to the left of us, and it turned out we were running towards the second explosion. We were right in the middle of the two bombs and we didn't know whether there would be another one. My daughter Shannon was going to run into another building, and I said no."

Eyewitness described the chaos. Anet Czyzewski, 23, a senior at Northeastern University, was coming out of the Prudential Center, across the street from the explosions.

"Everyone started screaming 'run', and all I knew was to run," she told the Guardian. "No one knew what happened – everyone's running and screaming and parents were grabbing their kids. I just thought I needed to get out of there.

"I didn't know what was happening. I didn't know if there was a bomb – they made it sound like there was a bomb in the Pru, I didn't know if it was a shooter. A bomb didn't really come to my head.

"As I'm running, a mom and daughter – she looked 10 or 12 – were running, and the mom fell because there were so many people. And the guard started screaming, and I was behind them, and selfishly I just kept running because I didn't know what to do. I was so scared."

Bruce Mendelsohn was on the third floor of an office building directly above the explosions. From the window, he snapped a picture of the aftermath, which went viral nearly instantly.

"I saw, heard, felt and smelled the explosions," Mendelsohn told the Guardian in an email. "They blew me off the couch I was sitting on. As a veteran, I know what these things are. I rushed outside to help. I saw many casualties, most with lower extremity wounds. It was chaotic, bloody and terrifying."

Down on the street, as spectators scattered, police officers, medical workers and some of the runners ran towards the blast sites to help the injured, pulling aside the twisted metal of the crash barriers that lined the race route to reach them. They were greeted by a scene of carnage – dozens of people on the ground, many with horrific wounds, including severed limbs. Blood was smeared across the sidewalk.

Roupen Bastajian, a state trooper and former marine from Rhode Island, had just received his medal for finishing the race when the first explosion struck. He immediately ran to help. "We started grabbing tourniquets and started tying legs," he told the Associated Press. "At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing.

"These runners just finished and they don't have legs now. So many of them," Bastajian told the New York Times. "There are so many people without legs. It's all blood. There's blood everywhere. You got bones, fragments. It's disgusting. It's like a war zone."

A medical tent, set up near the finish line for exhausted runners, suddenly became a facility for treating the blast victims.

Laura McLean, a runner from Toronto, told CBS that people who were "really, really bloody" were being pulled into the tent.

"They just started bringing people in with no limbs," said Tim Davey, a runner from Virginia. He and his wife, Lisa, said they tried to keep their children's eyes shielded from the scene before them, but "they saw a lot."

"They just kept filling up with more and more casualties," Lisa Davey told AP. "Almost everybody was conscious. They were very dazed."

By that point, more than 17,000 of the runners had finished the race, the oldest and one of the most prestigious marathons in the world, but thousands of others were further back along the course. The race took place on Patriots' Day, which commemorates the first battles of the American Revolution, at Concord and Lexington in 1775.

Hunter Felt, a freelance writer, had planned a typical Patriots' Day. "I was about to leave. I was planning to go to the Red Sox game then go to the finish line of the marathon. It's what a lot of people do."

He was diverted by work and didn't go, but believes the attack was aimed at those who follow that path. "It was designed to hit as many people as possible".

In the immediate aftermath, runners struggled to reach loved ones. Cellphone service was taken down shortly after the attack for fear mobile devices could be used to trigger other bombs.

Eshleman, an associate professor of psychology at Wagner College in New York, had travelled to Boston with four friends, and was assisting a blind runner around the marathon. She managed to text her boyfriend Nick Richardson back in New York, and he notified their friends via Facebook that she was OK. "Thank God for Facebook," she said. "I was so lucky."

The city went into high alert, imposing a no-fly zone above the scene, and briefly suspending flights into local airports. Police advised people to return to their homes or hotels and to stay indoors, while teams checked trash cans and bags left at the scene for explosives.

Amid speculation over the cause of the explosions, police commissioner Ed Davis said it was still too early to describe them as a terrorist attack but said they were the result of "two powerful devices".

Davis said he had no specific intelligence that "anything was going to happen at the race."

Police detonated a device found along the racecourse, but it did not appear to have been an explosive device, Davis said at a news briefing.

"We're encouraging people to not go out. If they're in hotels: stay in their rooms," Davis said. "After this incident occurred, there were a lot of people running from the scene, [and] a lot of them deposited bags and parcels," he said. "Each one is being treated as a suspicious device. At this point, we haven't found any more devices."

With authorities in Boston scrambling to investigate the source of the bombs, police in New York and other major cities said they had stepped up security at hotels and other "prominent" buildings. In London, police said they would step up security around the city's marathon this weekend.

Many of the injured were taken to Massachusetts General Hospital. On Monday evening, police wearing camouflage uniforms and carrying assault rifles guarded the main entrance. Inside were 22 people injured in the blasts, including six who were in critical condition and undergoing surgery, according to Alasdair Conn, the hospital's chief of emergency services.

At least four of them had arrived with traumatic amputations, he said.

Conn said that blast victims sometimes have internal injuries that are not immediately apparent, so doctors were planning to determine the extent of their injuries during surgery. The hospital had a special area set aside for families, he said.

On Monday evening, many eyewitnesses were struggling to come to terms with what they had seen. "I'm still kind of in shock," said Czyzewski. "I think what's really sad is that I didn't know what it was. That made it a lot worse. People tend to freak out instantly and that's why it made it worse for me and everyone there."

But she praised those who had rushed towards the blasts to help. "There are those people that instead of running away from it, ran to it to help people, which is amazing. I could never do that, and I'm just happy that there are people that who care about others and will go in danger to help others."

Eshleman agreed. "There was a feeling that everyone was doing everything they could to help," she said. © Guardian News and Media 2013