Armed police detain a man, and seize a gun and knife after being called to Crawley College.
A man has been detained and two college staff injured after reports of shots being fired on a campus.
Police found a knife and a firearm as they arrested an 18-year-old man near Crawley College at about 15:20 BST.
Neither of the injured are thought to be seriously hurt, and staff and students have been evacuated from the college, Sussex Police said.
Counter-terror police are working with local officers and “all lines of enquiry are open,” the force added.
Student Oliver McPherson, 18, who was at the college, said the “frightening experience” had left him feeling “shaken up”.
“It was quite hectic and it seemed liked something out of a game, like Call of Duty,” he said.
“Everyone was just shouting and screaming and bangs in the background, which some people assumed were gunshots.”
Teachers told students to “get under the table and shut the door, just as a precautionary measure,” he said.
Students later left the college to a “large police presence and police shouting and asking people to show their hands and evacuate the building in an orderly manner,” he added.
Ch Supt Howard Hodges said two members of college staff had “suffered minor injuries and presented themselves to a walk-in medical centre”.
“No one else has presented themselves to us with injuries, but if you need medical assistance please contact the ambulance service or speak to an officer,” he added.
Roads around the college were closed and the public was urged to stay away from the area, he said.
Police said that officers had searched a property, about a mile from the college, in connection with the case.
By Sam Harrison, reporter, BBC Radio Sussex
There are lots of worried parents and teachers stood outside, peering back into the college grounds as they await more information.
As I look down College Road there is sea of flashing police lights and there’s at least 20 police cars here, some marked, some unmarked.
Students are waiting on the pavement nearby. Everyone is quite anxious, everyone is awaiting for more information on what’s been quite a shocking day for people here in Crawley.
A group was seen being escorted out of a college building, and two helicopters have been patrolling the area.
Images on social media show police cars blocking the streets around the college, and scores of armed officers outside the building.
Chichester College Group, which runs the college, said it was not aware of any serious injuries, adding: “Our priority remains the safety of students and staff.”
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