An innocent eight-year-old boy reduced to tears by bigots who believed it acceptable to hurl vile tirades at a young child.
His crime? The colour of his skin.
Sonko can clearly recall that day when the youngster from a poor estate on the fringes of Paris turned out at a football tournament in a more affluent area.
The success of the team from the wrong side of the tracks didn't sit well with those who wanted glory for their own.
But the moment that jealously veered into the realms of a much uglier world has left an indelible mark on the 29-year-old.
'I was really young – about eight,' said Sonko.
'My team from Paris went to a tournament. The teams were better than us but we went to the final.
'There were lots of Africans and a whole mixture of other nationalities in our team.
'Because we were bigger and looked different the parents accused us of lying about our age and kept asking to see our passports.
'I came from a bad neighbourhood.
'They were saying there were too many coloured people in the team and are they really the right age?
'They said they didn't believe we were French.
'We all cried for the whole of the first half and were losing 2-0 at half-time. I remember crying at half-time because we were getting abused by the parents.
'They wanted their kids to win and were trying to do everything.
'Our manager said they were accusing us of things that we all knew weren't true.
'So the only way we could prove something was on the pitch.
'It was a case of "okay, if you think we are rough then we are rough".
'We ended up winning the trophy and before we had been given it everyone had left.
'But we turned it around and won and were really pleased. It's something that has stayed in my memory.'
Suffering at the hands of racists back then has left an indelible mark on the man who arrived on loan from Stoke in August.
Fortunately, the strides made against such mindlessness means the world knows there is no place for that kind of behaviour in civilised society.
Sonko is of no doubt England has nothing like the kind of problem which still blights some countries.
But it wasn't so long ago ugly chants and taunts were commonplace on the terraces around the country on a matchday.
Sonko is glad the focus is now on the colour of a player's shirt and not his skin but that doesn't mean we don't have to remain vigilant.
'I don't get any racism here,' said Sonko.
'Not since I came to England. We are lucky the world has moved on and people understand children are children and footballers are footballers.
'That's the case no matter what they look like or what colour their hair or skin is.
'But if it's not present we have to work against it still.
'We hear a lot about it in other countries and it's quite disappointing.
'We come together to play and we speak only one language and that's the ball.
'I can't understand people coming and focusing on people's ethnicity.
'Fans and players should look at the shirt. If they try for their team they should appreciate it.
'One day a player is in another team and the next he is in your team.
'I don't see the point of it. If you are a true fan you are not racist because anyone can play for you one day.'
As Pompey ambassador for Kick It Out, football's campaign for inclusion and equality, Sonko has become a figurehead for the club on the subject of guarding against racism since his arrival.
He was helping ensure the focus on that problem remains ahead of their One Game, One Community action to be held from October 14-26.
Sonko was speaking at a get-together organised in conjunction with Pompey Sports and Education Foundation where youngsters got to meet and fire questions at one of their heroes.
He is under no doubt at the importance of his role in both keeping the Kick it Out campaign in the public eye and his duty to spend time with the Blues' young fans.
Sonko said: 'I remember being a footballer at a young age – that is when you really start enjoying it.
'There are a lot of things and a lot of consequences which can make you a professional footballer and sometimes it can hurt.
'So the first part is to make sure you enjoy your football.
'I remember when I used to see professional footballers play and then they would come and talk to us.
'When I was growing up in France there were players we used to watch week-in, week-out.
'They used to play football and come and have a laugh with us.
'You see them and think "wow, I can kick a ball with them".
'There were people I looked up to as a youngster and it was great to spend time with them, so I know it's a big thing.'
The News, Portsmouth
