A murder probe was launched after police were called to reports of a knife attack in the Romford area by the ambulance service at about 9.25pm on Friday.
Jodie Chesney was pronounced dead at the scene, near St Neot’s Road in Harold Hill, just over an hour later.
No arrests have been made.
Posting on Facebook on Saturday, Jodie’s grandmother Debbie Chesney said too many young people were having their lives “cut short by needless violence”.
She called for anyone with information into the attack on her youngest granddaughter to contact the police.
Ms Chesney added: “This was our youngest granddaughter.
“How have we come to this point where kids can’t have a walk in a park without suffering an unprovoked attack?
“If anyone knows anything about this please contact the police with information.
“We don’t want anyone else to go through what our family is suffering right now.
“This has to stop, there are too many young people having their lives cut short by needless violence.”
The teenager has not been formally identified but the Metropolitan Police said her next of kin have been informed.
The death brings the number of homicides in London this year so far to 18.
On Tuesday, Che Morrison, 20, described by his family as a “loving” young man, was stabbed to death outside Ilford station in east London.
In the latest incident, a mother-of-three who tried to help the teenage girl in the park was “shocked by the amount of blood”, her husband said.
Paul Farenden, 54, said his 49-year-old wife Teresa had rushed to the scene near their home to see if she could help after she heard screaming.
He added: “My wife asked if everything was alright and one of the boys said ‘no, my girlfriend has been stabbed’.
“She put her in the recovery position and waited for the ambulance to turn up. I think she tried to resuscitate her.
“She thought it was all over then and it would be alright.”
But Mr Farenden said they later heard the teenager had died.
He said of his wife: “She is dealing with it better than she was last night. She was shocked with the amount of blood there.
“Normally you do not see anything like this in the park.
“Hopefully they will get someone for it. This just should not happen.”
Acting Detective Chief Superintendent John Ross, of the East Area Command Unit, said: “I want to express my deepest sympathies to this girl’s family and friends. Her death is a tragedy.
“I can reassure them and the whole community that we are doing everything possible to identify and bring to justice the person or persons responsible.”
Neighbours said young people often “hang out” in the park, and if they are too loud they are usually moved on without any trouble.
Hairdresser Ellie Best, 17, said she and her family had moved to the area for the “good strong community” life in Harold Hill.
A “shocked” Ms Best said: “No-one should have to get a call to say that their child has been killed.
“It is becoming more and more like central London here. Children did not fight or anything and you did not hear of people being mugged. There has never been knife crime here before - it is just in the last six or seven months.
“I worry for the younger youths. Police need to talk to them about the dangers of carrying knives because the message is not getting through.”
A small group of people cried and hugged each other after leaving flowers at the cordon with the message: “We love you forever in our hearts.”
Another message attached to a floral tribute said: “You are so strong. We will always remember you.”
The park, which has a playground at its centre and is known locally as Amy’s Park, has been cordoned off with police tape and is being guarded by officers for the forensic investigation.
Councillor Paul McGeary, who represents the Gooshays ward where the incident occurred, arrived with fellow councillor Tele Lawal to lay flowers at the scene.
Mr McGeary spoke of his “shock and horror” that the killing had happened in the semi-rural outer London borough.
He said: “This is not something that happens here and I am just completely surprised.”
He could not say if the death was gang-related, but described it as “tragic”.
Ms Lawal said the park, which she herself used to play in, was “not an area where you will frequently see violence like this”.
The 22-year-old added: “It is going to shock our community but it just shows the strain that is happening with young people, with our police and the resources we need in our community to tackle violence like this.”
Anyone with information is urged to contact police on 020 8345 3775, tweet @MetCC or call Crimestoppers anonymously on 0800 555 111.