After an EDL rally in Whitehall was dispersed earlier, supporters of the far-right group are now running through Leicester Square chanting "English till I die" and throwing bottles at police.
Police are holding the protesters in Leicester Square and closing the pubs in which they had been drinking.
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The Prime Minister will say Moscow's brazen flouting of international law represents a threat to the basis of democracy across Europe.
In some cases, victims of violence and other serious crimes face long delays as forces are unable to dispatch officers promptly.