Part of Manchester city centre was cordoned off and a man detained after two suspicious devices were found.
Greater Manchester Police (GMP) evacuated Piccadilly Gardens and a bomb disposal robot was used to examine the packages.
GMP said it was keeping an “open mind” over links to a third device, which led to Oldham Library being evacuated.
Police said none of the devices was viable and a 26-year-old man was in custody.
The suspect was questioned on suspicion of a bomb hoax offence, possessing a prohibited weapon, possessing a bladed article and possessing an offensive weapon.
A cordon was put in place at about 13:30 BST, but Piccadilly Gardens was re-opened to the public at about 20:00.
Small cordons remain in both locations as inquiries are carried out.
A loud noise heard at 18:15 was caused by bomb disposal officers as part of their assessment to ensure the package was safe, police said.
Transport services, including the Metrolink, were affected as a result of the cordon.
Witnesses described seeing police going through a bag on the ground.
Police said Oldham Library, eight miles north-east of the city centre, had also been evacuated as part of their inquiries after reports a “suspicious device” was found there.
The device was deemed to be “non-viable”.
A police spokesman said: “Officers are keeping an open mind and are exploring the possibility the incidents are linked.”
Supt Dave Pester added: “The investigation will look at every available piece of information so we can understand the motivation behind it and the circumstances that surround it.”