Speaking outside her Downing Street residence, May said the attack was "a reminder that terrorism, and hatred take many forms and our determination to tackle them must be the same, whoever is responsible".
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May said the attack was "every bit as sickening as those that have come before", referring to three Islamist-inspired attacks in London and Manchester this year that have killed 35 people and injured around 200.
The attack "targeted the ordinary and the innocent... this time British Muslims as they left the mosque," she said, adding that police would provide any additional protection needed for mosques.
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"This was an attack on Muslims near their place of worship and like all terrorism, in whatever form, it shares the same fundamental goal: it seeks to drive us apart," she said.
"There has been far too much tolerance of extremism... including Islamophobia," she said, announcing the creation of a new commission to fight extremism in the same way as racism.
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