Police have recovered a weapon from the scene in Edmonton but would not confirm reports that the knife was a machete.
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Through the day, most of the cloud should break up to leave fine conditions for many with spells of sunshine.
The Eiffel Tower in Paris was plunged into darkness, with the Burj Khalifa in Dubia emblazoned with the Union Jack.
The major party leaders' campaigns for the General Election will remain suspended on Wednesday following the Manchester terror attack.