Debris from an explosive device was found close to the Central Mosque in Wolverhampton, a West Midlands Police statement said.
"The find was made late last night (Thursday) by officers and army bomb disposal experts as they searched land in and around the building on Waterloo Road," the statement said.
The mosque was evacuated last evening but has been reopened for worship following the discovery.
Information about the device, which police believe was activated on June 28, was uncovered as part of an investigation into recent attacks at nearby mosques in Walsall and Tipton.
The arrests came less than 24 hours after police issued CCTV images of a man they urgently wished to trace in connection with the investigation.
Last Friday, residents were evacuated after a blast near the Kanzul Iman Masjid in Tipton left nails and debris scattered outside.
No one was injured but some minor damage to property was reported.
A small component part of the suspected nail bomb was found in a garden nearby and specialist army disposal officers were called to the scene to carry out a controlled explosion.
No injuries were reported as a result of the blast, which caused minimal damage to a wall near the mosque in Rutter Street on the evening of June 21.
Although residents in the area heard a loud bang at the time of the explosion, the incident was not reported to the police until the following day when parts of small home-made device were found by a worshipper at the mosque, sparking a major operation involving Army bomb disposal experts.
