The 32-year-old said he momentarily believed the explosions were thunderclaps, before he remembered it was a perfectly sunny day outside.

He went to the restaurant window, which overlooks one of the side streets off the main marathon course, and saw masses of people start to run past.

He immediately began to worry, and attempted to contact other Australians who were in Boston to compete in the world's oldest annual marathon.

"I just got back to my hotel, which is the race hotel, which is in lockdown, and security kind of everywhere and all that," Fearnley told ABC 702.

"But mate, yeah, it's ... a bit of a terrible thing, you know, it's a bit of a shocker. You don't expect this sort of stuff to happen when you're sitting outside."

He said Australian marathon champion Robert de Castella was also in Boston, where he was mentoring a group of indigenous runners.

LISTEN TO FEARNLEY DESCRIBE THE SCENE IN BOSTON. WARNING: IMAGES MAY DISTRESS SOME VIEWERS

He said De Castella was safe, however one of his runners, a woman from near Katherine, was still out on the course and they had been unable to contact her.

"She's the one person that we haven't been able to track down," Fearnley said.

"You hit everyone, as soon as you find out, you kind of hit every Aussie that you know is here and the only one that we're waiting for is the last [of the] indigenous marathon group, so hopefully all is good mate. It's not all good, it looks shit, it looks terrible ... apologies ... it looks like a real terrible event."

He said police had flooded Boston's streets.

"Mate, the whole area's on lockdown. My hotel's on the finish line, about 100 metres past the finish line, there's troopers everywhere, there's coppers everywhere and even the marathon crew now ... [have] brought in their own security now."

He said there were many people still out running the course, perhaps unaware of what had happened, but the finishing line had been shut down.

"You're not finishing, you're not getting near that finish line right now, there's too many people there that need to actually do their job and to heal people and if there's something that's bigger than sport or bigger than a day in itself, you know, and this is one of those times."

Fearnley said he hoped that "people don't lose their lives and families get to be with each other".

A spokesperson for the Department of Foreign Affairs and Trade said there were no immediate reports of Australians injured or killed in Boston.

"The Australian Consulate-General in New York is making urgent inquiries to determine whether any Australian citizens may have been involved in the explosions that have occurred at the Boston Marathon on Monday afternoon [Boston time]," the spokesperson said.

"Australian runners were scheduled to compete in the event.

"At this time we are not aware that there are any Australian victims."