Officers believe the two attacks, which were carried out on separate occasions, were committed by the same man.
At around 11:30hrs on Monday, 14 September, a girl, aged in her early teens, was walking through Central Park in Dagenham when she was approached by an older man who sexually assaulted her.
The second offence took place on Monday, 21 September, when a girl, aged in her mid-teens, was sexually assaulted walking through the same park, again at around 11:30hrs.
The suspect is believed to be aged between 45 and 60 years old, 5ft 5ins tall, slightly overweight, with a balding head and tanned skin.
On both occasions, the man left the park straight after the assaults and police believe he may have been disturbed by potential witnesses in the park.
Officers will be stepping up patrols in the area but would like to reassure the public not to feel alarmed.
Detective Constable Neil Godwin, from the East Area Command Unit, said: “These were frightening incidents which, understandably, left both girls very distressed and upset.
“Both assaults happened mid-morning, when plenty of people were around, and I would urge anyone who saw anything to get in touch.
“We would also ask the public to remain vigilant, but not alarmed, and to get in touch if they see anything suspicious.”
Any witnesses or anyone with information is asked to contact police on 101, quoting reference Cad 2737/14Sep.