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."
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.