Around 36,000 runners took part in the London event, just days after three people were killed and more than 170 injured after two explosions at the Boston Marathon.

Mr de Castella, who returned to Australia on Thursday after taking part in the US event, described the turnout as the “greatest deterrent” for possible future attacks.

“It's actually going to make us a stronger, more resilient, more determined community and that's what we've seen with London,” he said.

“The horrific bombings in Boston only fuelled the human spirit.”

Mr de Castella had finished the race and was in his hotel when the he heard the blasts, which he said sent vibrations through the room.

Speaking at an event in Canberra on Monday, he said the experience still seemed surreal.

“The marathon itself is an incredible community event that brings people from all over the world, from all ages and backgrounds and religions together to participate in a personal challenge,” he said.

“To have someone abuse that and violate it in such a way, by killing and maiming people at the finish… is just an incredibly abusive and horrific thing to contemplate.”

As the director of the Indigenous Marathon Project, Mr de Castella had travelled to Boston with Darwin runner Emma Cameron, who was only 1km away from the finishing line at the time of the explosions.

He described her handling of the event as “incredible”.

“We choose to run the marathon… because we want to test people,” he said.

“For Emma to not only do that and then also have to cope with the emotion of the bombing was another level of personal challenge. I was just so proud of her, of the way she handled it all and came through. I've spoken to her since she got home and she's settled in really well.”

The manhunt for the two bombing suspects, brothers Tamerlan and Dzhokhar Tsarnaev, resulted in the death of a police officer. Nineteen-year-old Dzhokhar remains in a serious but stable condition in hospital after the prolonged shootout which killed his 26-year-old brother.