Police are carrying out an investigation on land near Netherfield Lagoons.
They were spotted near the Stoke Lock side of the nature reserve.
People are being encouraged to stay away from that area while an investigation continues.
Readers reported seeing emergency services speeding towards the scene through Netherfield around lunchtime.
Some people on social media said they had been turned away from trying to access parts of the nature reserve by police earlier this afternoon.
A police spokesman said: “We were called out today at 12.10 to an incident and an investigation is ongoing.”