Mrs May was outside Downing Street after chairing a meeting of the government's emergency Cobra committee. "I think at least eight or 10 people were injured".
The MCB's head Harun Khan said the van had "intentionally" run over people leaving night prayers held during the holy month of Ramadan.
London's Counter Terrorism Command is leading the investigation into the attack.
The imam of Muslim Welfare House - which is also a community centre - said a passing police van was flagged down after the attack.
The chairman of the Finsbury Park Mosque says the van crash that hit worshippers was a "cowardly attack" and urged Muslims going to mosques to be vigilant.
The deadly van-ramming attack at a mosque in London is "a painful illustration of why we must never allow hatred to breed hatred", Britain's chief rabbi said.
London police say a 47-year-old man has been arrested on suspicion of commission, preparation or instigation of terrorism including murder as well as attempted murder after a auto plowed into a crowd of pedestrians near a north London mosque.
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The start of the negotiations comes as work continues across the British government to prepare Britain for life outside of the EU. The session ended with a joint press conference in which the negotiators again stressed they were entering the talks positively.
Another terrorist turns a vehicle into an weapon in London, driving into a crowd of worshipers just leaving a prayer service. "And then the imam of the mosque actually came out and said: "Don't hit him, hand him over to the police, pin him down".
The Finsbury Park mosque was associated with extremist ideology for several years after the 9/11 attacks in the United States but was shut down and reorganized. Police are treating the incident as a terrorist attack.
Police said at this stage they believe the suspect acted alone. My thoughts are with the family of the man who has died and with all those who were injured.
A Metropolitan Police statement said that due to the nature of the attack, "extra policing resources have been deployed in order to reassure communities, especially those observing Ramadan".
"Like the terrorist incidents in London and Manchester, this incident seeks to erode our shared values of tolerance and mutual respect".
Another witness, who wanted to be identified as Abdulrahman, which is not his real name, told the Press Association: "I managed to get the driver of the van". Eight people were killed and dozens were wounded before police fatally shot the three attackers.
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