Mohammed Mahmoud, of the Muslim Welfare House, said the group gathered around a man later identified by British media as 47-year-old Darren Osborne after he careened into a crowd of people who had just finished Ramadan prayers.
The apparent attack, which London Mayor Sadiq Khan called a "horrific" act of terrorism, occurred at 12:20 a.m. local time (7:20 p.m. ET).
"Some of them tried to hit him, either kicks or punches," Mahmoud said. "By God's grace we managed to surround him and protect him from any harm. We stopped all forms of attack and abuse towards him that were coming from every angle."