Police are responding to reports of a "significant incident" at RAF Mildenhall.
The US Air Force base in Suffolk is reportedly on lockdown amid reports of a car trying to ram the gates.
A brief statement on the base's Facebook page said: "RAF Mildenhall locked down at 1pm today following reports of a disturbance on base.
"The base was locked down and emergency personnel are responding to the situation.
"Additional details will be provided as they become available."
Two businesses on the base confirmed they were no longer on lockdown after the security alert.