One detonated and set fire to a rubbish bin near the Bendigo Library in Hargreaves Street on Sunday morning.

The other, described by police as a modified pipe bomb, was found in a container in Chancery Lane.

Police prevented it from exploding by using robots to safely deactivate the device.

Sergeant Steve Rainy said police were confident the explosions were "moronic" pranks and not acts of terrorism.

"This is just stupidity at its best, particularly in the current climate with events around the country," he said.

"It's nothing short of moronic to come out the weekend before Christmas with any type of explosive device and think detonating that is fun.

"It could have had catastrophic effects for someone over the Christmas period."

Sergeant Rainy said police were "extremely confident" they would catch the perpetrators, and would examine CCTV footage to assist their investigations.

He said it was lucky no one was hurt by the devices.

If someone had been standing next to the rubbish bin when it exploded they could have suffered significant injuries, Sergeant Rainy said, and there was "no knowing" how much damage the pipe bomb might have caused.

He said the bomb disposal unit worked on the second device at 6am, using robots to empty the device of powder, which is being tested.

He said the devices, discovered between 3am and 4am on Sunday, were homemade.

"These are fairly simple devices - someone has certainly made them at home," he said.

Police would continue to lock down the area, he said.

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