Eyewitnesses said four members of Britain First, a small far-right political party, parked on a pedestrian crossing and entered the mosque a few hours ago.
They included the leader of the group Paul Golding, and a woman who was filming proceedings. The four appeared to be accompanied by a professional filmmaker who may not have been a supporter.
Some in the mosque initially thought they were members of ambulance support, since they were wearing green and had been directed to the mosque reception once they entered.
An eyewitness said: "They walked across the lobby where people pray, ignored a sign requesting people to take off their shoes, but didn't walk into the prayer room. They went to the mosque reeption and asked to speak to the Imam, who wasn't there."
The group then gave out leaflets and bibles before rushing back to their vehicle – they had left the door open.
Update: May 19 15:49 B.S.T: BuzzFeed has obtained CCTV footage of the incident.
As they were driving away a number of Police Community Support Officers approached. An eyewitness said: "They almost jumped a red light but managed to stop half way through."
Members of the congregation are now sitting calmly in the mosque's lobby. The overall atmosphere is relaxed and no one is particularly shocked or upset. One eyewitness said: "They were more brave than last time, when they just drunk outside the mosque."
We're going to return. It will continue until there's a big campaign, organised by Muslim leaders, against grooming within Muslim communities.
We condemn this type of behaviour. The mosque and centre is a place of religion and worship. Our doors are open to Muslims and non-Muslims but the way they went in today is unacceptable. The way they walked over prayer mats is nothing short of ignorant. They claim to represent the Christian faith. But we have very good relations with the Christians in the area.They're using the grooming issue as a guise to attack the Muslim community. If they were so concerned about grooming, we saw the sentencing of ten people in October for grooming. They weren't of the Muslim community. I would question their intentions.
Britain First was launched in May 2011. Golding, the chairman, has previously been a councillor for the British National Party.
It claims it seeks to protect "British and Christian morality", and campaigns against radical Islam, immigration and abortion.
Last week Golding claimed he was subjected to a police "inquisition" after giving a series of TV interviews in London.