The historic cathedral in Staffordshire was evacuated this lunchtime with staff and tourists ordered to leave the building.
It is believed a suspicious package was found at the cathedral.
Police have closed off Cathedral Close and evacuated residents from the area as they "review" the situation and search the building.
Officers have urged people to "remain calm".
Lichfield Cathedral has been evacuated over a
We are aware of a report of a suspicious incident reported at Lichfield Cathedral
Witness Kenny Wilding said: "Just evacuated Lichfield Cathedral where I working as someone went in with a package and came out without it."
The Very Reverend Adrian Dorber said: "Police are here now. They're continuing to search.
"They've asked the Close to be kept quiet.They're here in force basically for everyone's peace of mind."
Lichfield Police said: "Specialist officers are currently searching #Lichfield Cathedral after concerns about a man who was seen acting in a suspicious way."
Police have surrounded Lichfield Cathedral
Get Quotes on Home Insurance
Staffordshire Police said in a statement: "We are aware of a report of a suspicious incident reported at Lichfield Cathedral this lunchtime, and that the management at the location have evacuated the site prior to police attendance.
"We are carefully reviewing the report and have the appropriate response in place, we ask the public to follow the instructions of those managing the incident.
"We are not aware of any specific threat to the location, however we continue to ask the public to report any concerns in view of the current, national, threat level and as a precautionary measure officers are investigating the report.
"We will continue to provide updates as appropriate but urge our communities to remain calm."
Armed police have been seen in the city in recent days in the wake of the Manchester terror attack.
Lichfield Cathedral tweeted: "Our team evacuated the cathedral today after a suspicious incident and is now working with @StaffsPolice as investigations continue.
"We’re not aware of any specific threat & are working to reopen ASAP. Our Mr Turner exhibition will be closed today, unless advised otherwise."
Lichfield Cathedral Close closed-off to the public pic.twitter.com/LUvxK2lAn6
— Nick Brickett (@nickbrickett) May 30, 2017
The cathedral is currently hosting a JMW Turner exhibition and had finished the 12.30pm Eucharist service shortly before the evacuation.
Lichfield Cathedral is the only medieval three-spired Cathedral in the UK and is one of the oldest places of Christian worship, and the burial place of the great Anglo-Saxon missionary Bishop, St Chad.
Britain's terror threat is currently at "severe" meaning an attack is highly likely.
Armed police have been patrolling across the country and military personnel are being gradually stood down as the huge security operation launched after the Manchester bombing is scaled back.
Hundreds of troops were called in to support the police effort amid fears further attacks could be imminent.
The plan, Operation Temperer, was activated after the official threat level was raised to critical, the highest level, for the first time in almost a decade.
Over the weekend the assessment was changed back to severe, the second most serious rating.
More to follow…