An envelope containing a small amount of 'white powder' was delivered to Blackpool's Central Mosque and Islamic Community Centre earlier today.
Emergency services were called to Revoe Street shortly before 2.30pm.
The substance has been sent for analysis and an investigation is underway, a spokeswoman for Lancashire Police said.
Witnesses described seeing a number of emergency service vehicles close to the mosque, and said police officers had taped the scene off.
One said there was a 'major incident vehicle, an ambulance, and police vans.'
The incident appeared to be largely over by 5.30pm, with just one police car pictured near the mosque.
"The powder has been examined, and although it has not been identified, it's not believed to be harmful," a police inspector added.
"Thankfully nobody has been hurt."
However, resort leaders will still be alarmed by the drama, which was the second such incident in as many weeks.
Coun Tony Williams said he didn’t know who the envelope was addressed to, but said: “This can happen from time to time with all councils and public bodies, and at a time when we on high alert, we have to be very, very careful and very safe.”
The powder sent to the town hall, in Talbot Road, was deemed to be safe after part of the historic building was locked down. It resembled fish food, one official said, and was also sent to a lab for testing.