Hundreds of people have staged a protest in Trafalgar Square against the so-called bedroom tax.
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Protesters threw firebombs and took their anger out on shops with mainland Chinese ties.
Ms Truss says numerous countries including the US and Japan are keen to secure trade agreements with the UK.
A preliminary count of 84 per cent of the votes showed Evo Morales on top with 45.3 per cent, followed by 38.2 per cent for his closest riva