Rabbi Herschel Guck said the incident was an attack on "every single Muslim in the UK" but also "every single citizen in Great Britain because we are one nation".
Osborne rammed a van into pedestrians outside the mosque in Finsbury Park, north London, on Monday.
The suspect has since been identified by multiple news outlets as Darren Osborne, a 47-year old married father-of-four living in a Cardiff suburb who has openly voiced a hatred of Muslims.
Security Minister Ben Wallace said the driver was "not known to the authorities in the space of extremism or far-right extremism".
Pedestrians were targeted by an attacker driving a van early on Monday, with one man pronounced dead at the scene and nine taken to hospital.
Police confirmed one person has been arrested, with United Kingdom media reporting he was the driver of the van.
Ms Osborne said her son had never shown extremist views or behaviour.
Mahmoud said he'd just finished leading prayers when he learned of the van attack.
Locals massing at barriers, angry at police, people confused about what has happened.
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A witness, Abdiqadir Warra, said the driver of the van "drove at people" and that some of the victims were carried for several metres along the road.
The suspect, named as Darren Osborne, had been shouting earlier "I've done my bit" and "you deserve it", according to Toufik Kacimi, the chief executive of the nearby Muslim Welfare House community centre, who praised the imam's courage.
The family of the Finsbury Park terror attack suspect have described the incident in north London as "sheer madness" and said they are "devastated for the families".
Hundreds of people took flowers to Finsbury Park and gathered outside a mosque in the area on Monday night at a vigil held for victims of the attack.
TOP IMAGE: The white van used in the latest London terror incident, near Finsbury Park Mosque.
After meeting with the chairman of the mosque, Mohammed Kozbar, she stood with him to observe a short silence.
The Metropolitan Police report Osborne is now "being held on suspicion of the commission, preparation or instigation of terrorism including murder and attempted murder".
Deputy Assistant Commissioner Neil Basu said it was not yet clear whether the man who died at the scene was killed because of the attack. They also expressed frustration that attacks on the Muslim community have received little coverage or sympathy.
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