An unidentified group gathered outside the Bristol Jamia Mosque, the largest in south west England.
Bacon sandwiches were thrown at the mosque's doors and another report claimed bacon was hung outside the mosque.
Locals tried to intervene and police were called to the scene in Totterdown, Bristol after the incident at around 12.50pm on Sunday.
This morning, sympathy cards and flowers have been left outside the main entrance.
Rabia Zaheer, who witnessed the attack, said: "Pig meat was hung outside at prayer time and they hung their flag up.
"The worst thing is they decided to hang around and hurl abuse at the elderly men trying to pray.
"The locals did try to intervene and shout the scum down. Police are involved and the CCTV is being checked.
"Typical - they attack on a day when it's mostly elderly men at the mosque and no young ones.
"I've lived here all my life and nothing like this has ever happened before."
Bacon sandwiches outside the door to the Jamia Mosque
Local community leaders denounced the "cowardly" attack.
Abdul Raoof Malik, chair of Easton Jamia Mosque and a member of the Council of Bristol Mosques, said he was "very saddened" to hear of the incident.
"This type of incident is very rare in our wonderful city and we do not feel that this is anything more than a group of opportunists who found a spot to take a photo which indeed reflects their character and offensive cause.
"This is an isolated incident and we hope the local neighbourhood will help the police in their enquires, which I'm sure are happening right now.
"We are proud of the unity in our great city and we will not let a few individuals fuel any hatred towards us or allow any individual or group break the bridges it has taken years to make between us."
The Bristol Jamia Mosque
Bristol mayor George Ferguson wrote on Twitter: "I'm disgusted by cowardly attack on members of Jamia #Mosque today and stand with you to show #Bristol does not tolerate such vile behaviour."
Police Chief Inspector Kevan Rowlands said: "Behaviour of this kind is totally unacceptable.
"Our communities have the right to live and worship peacefully without fear of being targeted for their race or religion.
"We take all hate crime incidents extremely seriously and we are working closely with the mosque to ensure the offenders are held to account for their abhorrent actions."
Mayor of Bristol George Ferguson is disgusted by the attack
Bristol Jamia Mosque, which can accommodate 840 worshippers, was the first to be opened in the city in 1968.
Police said two men and one woman were involved in the incident, which they are treating as hate crime.
A 34 year-old man was arrested and remains in custody.