Witnesses heard 'incredible bangs' which awoke residents and members of Pep Guardiola's travelling squad at the Radisson Blu in Dortmund.
A gang of 17 ultras set off the first batch of 'industrial strength' fireworks at 2.45am which sparked City's security staff to patrol the area.
Then at 4.30am around half a dozen of the group returned and set off a second explosion in a different area near the hotel, again waking up those inside.
Police were called to inspect the incidents, but it is claimed City's security were forced to foil a THIRD attempt at 5.30am.
The club's staff are said to have spotted members of the gang returning and managed to chase them away.
A source told the Daily Mail: "These were not just bangers. We are talking industrial strength fireworks.
"The bangs were incredible. It felt as if the hotel shook."
City meet BVB tonight in the second-leg of their Champions League quarterfinal defending a 2-1 lead.