(Warning: Some viewers may find the content of the video distressing.)
Robin Hogg, who has spoken to the media for the first time, was shopping with 14-year-old Stephen Parker on the Cavehill Road when one of 20 IRA bombs exploded in Belfast on 21 July 1972.
"As I came out of the Co-Op I was hit by a flash and a large explosion followed by smoke and rubble, and dust," he said, quickly realising that a bomb had gone off.
He escaped the blast unhurt.
"There was a lot of screaming and crying... There was general mayhem."
Mr Hogg could not find Stephen, who had gone into a shop across the road, so he went home but later found out his friend had died in the blast.
"In many ways I'm glad I didn't find Stephen because of the condition he was found," explained Mr Hogg, who said he had heard bombs going off during the day, but added "You don't think it's going to be on your doorstep".
It's something that I do not think I really have ever got over. I vividly remember that day. The shock of it. It's something I have relived many times since.
"I've remembered him not just on Bloody Friday. The date is significant but I don't think as a close childhood friend you forget and remember just for one day," he said.
Nine people were killed and 130 injured across the city that day, which has become known as Bloody Friday. Stephen was one of three people killed in the bomb on the Cavehill Road, while six people died when a car bomb exploded at Oxford Street station, one of the Belfast's busiest bus depots.
The explosions caused widespread confusion and panic in Belfast as the bombs - which all detonated within an hour and a quarter - could be heard throughout the city. As people moved from one area to another, they were aware that there was no safe place in which they could escape the possibility of being caught up in another explosion.
John Knox's fire crew was based at Castlereagh, but they were drafted in to help at Oxford Street bus station, where a fire was still raging when he arrived.
He helped to put out the blaze, while he said his colleagues "were directed to look for body parts, pick them out from the debris and get them put in plastic bags".
He said he soon realised some people were going "to get very bad news". "Somebody's going to be without a father, somebody's going to be with a husband, a loved one", he recalled.
"They did not know at that time what was going to hit them, but I did and that hit me as well," Mr Knox explained.
I lost a great friend. I lost a soul mate and I lost part of myself in many aspects.
Paramedic Andy Jenkins watched as body parts were placed on a tarpaulin following the blast at Oxford Street.
"There were bits and pieces and we were standing looking, the press was there with their cameras and one of my colleagues said 'stand back, stand back and let them see what the bombers have done'. He wanted the bombers to see what they had done.
"It's revolting to think that people could do that," he added.
Although Mr Jenkins said the clear-up operation was just part of his job, and "you just have to get on with it", he told UTV he was deeply affected by what he saw that day.
"It's when you get home at night and you're in bed, it's all in your head. You don't sleep the whole night, you're waking up and seeing it all over again.
"No matter how much you try it's just going through your mind over and over again."
On Bloody Friday, the IRA's Belfast battalion issued a statement accepting responsibility for all the explosions in the Belfast area, and claimed that the Samaritans, the Public Protection Agency and the press "were informed of bomb positions at least 30 minutes to one hour before each explosion".
However, it is thought the paramilitary group had overestimated the police, fire and ambulance service response, as the emergency services dealt with 20 explosions and a number of bomb scares in a geographically limited area over a short period of time.
On the 30th anniversary of Bloody Friday, the Provisional IRA released an apology for the attacks, in which the group said it offered "sincere apologies and condolences to [the] families" of "non-combatants".
"While it was not our intention to injure or kill non-combatants, the reality is that on this and on a number of other occasions, that was the consequence of our actions," they said.
For those caught up in the horror of Bloody Friday, they will never forget what they saw.
Forty years on Mr Hogg, who remembers his friend Stephen as a "smiling, caring, happy-go-lucky person, added: "Over time, the memories fade, but there have been instances where it comes back and it's just as vivid as ever."