He previously funded advocacy group Cage, one of whose leaders described Mohammed Emwazi - who became known as the terrorist Jihadi John - as a “beautiful young man”.
Sara Khan, the commissioner for countering extremism, has accused Mend of being a divisive organisation with some staff who have engaged in hate.
The Civil Service seminar is due to take place on Thursday.
David Toube of the think tank Quilliam said: “Civil servants should be actively engaged in disrupting the activities of this organisation, not hosting its founder as an honoured guest.”
On Monday The Times reported that the event had been scrapped. The government said: “This invitation should never have been issued.”
