One person has been arrested after a bomb scare on a Canterbury bridge.

Police are working on clearing the Selwyn River Bridge in Glentunnel - after a man parked a silver bus on it he claimed was full of explosives.

Police say he's now in custody.

Wairiri Road and the bridge should reopen shortly.

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Local man Richard Dawson was first at the scene and spoke to the man through a window of the bus.

He saw the bus stop in the middle of the road and got out of his car to assist them across.

"When I went up, this guy was very agitated and he said he had a weapon."

Dawson tried to talk to the man some more and calm him down, but it didn't work.

"He just looked at me and said 'best you go away, I'm armed and the bus is full of explosives.'

"I looked at the door and saw a bit of wire tied around the door, but whether that's anything I'm not sure.

"I just turned around and ran."

Dawson said the man is still inside the bus and two trucks have blocked both sides of the bridge.

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"He's very agitated, sitting in the driver's seat and flashing his lights."

Dawson said police have since arrived and taken control of the scene.