The 44-year-old man, who is believed to have sympathies with animal rights' groups, had already planted two devices on commercial premises in the city, which were defused by army bomb disposal squads.
Detectives from the South East Regional Crime Squad arrested the man, who is of no fixed address.
The arrest was the culmination of what police described as several weeks of round-the-clock surveillance.
Police said they believed the man was about to carry out a campaign against companies involved in animal research.
The man was arrested in the Broadmead shopping centre in Bristol after police had seen him plant two devices in shops, a police spokeswoman said.
They found four more armed devices in his coat, she said.Both shops were thought to be involved in animal research.
Fifty police officers had been watching the man over several weeks, said the spokeswoman.The regional crime squad was called in after a tip-off about the man's activities, she said.
The man's movements over large areas of the country meant that the scale of the surveillance needed was beyond the scope of one force. The Midlands Regional Crime Squad and Avon and Somerset police assisted the squad in the inquiry.
"It was a fairly well planned exercise on the man's part," said the spokeswoman.Reuse content