The Metropolitan Police said officers on patrol in Regents Park Road were flagged down by the victim, who reported that she had been raped in the park, at 10.50pm on Friday (April 11).
The incident happened on the same night that Royal Parks, which manages Primrose Hill, was launching its weekend summer closures.
Royal Parks had previously announced that gates to the popular open space would be locked from 10pm until 6am on Fridays, Saturdays, and Sundays until October.
It has since emerged that the park gates were not locked at 10pm, but an hour later.
A spokesperson for residents group Primrose Hill Watch said it had two concerns – the failure to close the gates on time, and whether the attacker was known to the victim or a rapist was on the loose.
They said: “We were obviously waiting for the actual park to be shut on Friday and waiting for that to happen and noticed it wasn’t happening.
“It’s important people know the park was open when it happened. The park was shut late the first night.
“The community also needs to know if the assailant was known to the victim. That is really important.”
The Met says the victim was taken to hospital by London Ambulance Service, and is being supported by specialist police officers.
There have not been any arrests.
Royal Parks did not respond to comments regarding the closing of the gates.
A spokesperson said: “We are deeply saddened to hear of an incident involving a 16-year-old girl on Primrose Hill on Friday 11 April.
“Our thoughts are with her and her family. The incident is being investigated by the Metropolitan Police.”
Anyone with information should call police on 101 or message @MetCC quoting CAD 8380/11APR.
Alternatively, contact the independent charity Crimestoppers anonymously on 0800 555 111 or visit crimestoppers-uk.org.
