- Video report by Health and Social Affairs Correspondent Rebecca Barry
The father of Jodie Chesney, who was stabbed to death in a London park last Friday, has said her attacker has "destroyed something beautiful".
His comments come on the same day people from her community gathered to march in memory of the teenager.
The 17-year-old was with friends near a children’s playground in Harold Hill, Romford, when she was approached by two males and stabbed in the back in a seemingly motiveless attack.
She was pronounced dead an hour after emergency services were called to the scene at 9.25pm.
Speaking of Jodie's attackers, her father Peter Chesney said: "You'll never know the pain you've caused and the beauty you've taken away.
"They've destroyed something beautiful here...It's just a disgrace."
A man was arrested in Leicester on Tuesday in connection with the murder of Ms Chesney.
Appealing for people to come forward Mr Chesney said: "Someone knows who did this. Somebody knows who did this. Just do the right thing.
"We need this, Jodie needs this, it's just horrendous," he continued.
Mr Chesney, speaking alongside his partner Joanne and daughter Lucy, said Jodie, who he described as a "beautiful soul", was killed on his birthday.
"The last time I spoke to her was Friday morning. She got up to wish me happy birthday before I went to work," he said.
He added her friends had described the attack to him and how they and her boyfriend attempted to console her after she was stabbed in the back.
A post-mortem carried out on Sunday established the cause of the Ms Chesney's death was trauma and haemorrhage.
The father branded the stabbing as "ridiculous" and he didn't know why anyone would want to hurt his daughter.
"There is nothing that Jodie would have done about this to have deserved this. It's just ridiculous, I don't know why you'd do it."
He added: "My daughter was sitting in the park, minding her own business and this happened to her, and this is not okay."
Jodie was an Adventure Scout and school friends said students at Havering College would be wearing purple this Friday in honour of Jodie and her Barking & Dagenham scout troop colours.
On Thursday evening, around 1,000 people marched through Romford calling for an end to knife crime and delivering a petition to the police asking for more action on the issue.
People in the town spoke about how change is needed to stop the spike in violent incidents in the capital. 11 people have died from incidents linked to knives since the start of 2019 - Jodie is among them.
Among the sympathy was fear and anger. One woman who joined the march said: "I think the police are scared as well, I think everybody is scared. Enough is enough. A lot more needs to be done. We're not going to take it no more."
- Video report from ITV News correspondent Geraint Vincent
They wore purple ribbons and carried purple balloons in memory of Jodie.
Mr Chesney said: "All of her friends are dying their hair purple, we've got purple ribbons made as well to remember Jodie.
"It is nice, it's not going to bring her back but I'm happy that she will be remembered like this," he added.
Mr Chesney also said there are "loopholes" around carrying a knife and called for maximum sentences for those carrying a weapon.
He said: "I think there should be a maximum sentence for people carrying a knife, that's the only way we are going to be able to stop this."