A main route between England and Wales was closed after a man climbed a bridge and flew a drone from the top.
Traffic was stopped on the M48 - the older of two Severn crossings - because of "concern for welfare," police said.
The man, in his 20s, came down voluntarily from the 47m (154ft) bridge tower and was arrested on suspicion of causing a public nuisance.
Highways England said it was deeply concerned and that "a person has put their life at serious risk".
"The incident was quickly spotted on our security cameras and reported to police and thankfully there was no injury or worse on this occasion," it said.
"Appropriate security is in place on the bridge, we are liaising with Avon and Somerset Police and will be undertaking investigations to determine if any damage was caused during the incident."
Police said: "Officers attended the M48 Severn Bridge at 08:10 this morning after concerns were raised for a man who appeared to have climbed one of the towers and was flying a drone off it."
The original Severn Bridge opened in 1966 and until recently was a toll road, but it is now free to cross.
Drone activity recently caused the closure of Gatwick Airport for several days.
About 1,000 flights were affected during the chaos between 19 and 21 December when drones were seen near the runway.
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Two people who were arrested were released without charge and police have made no further arrests.
Sussex Police has renewed its appeal for help to find "the criminal whose activities led to widespread disruption".