The MP Pamela Nash has tweeted a picture which from Portcullis House shows the scale of the Anonymous protest in Parliament Square.
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A 23-year-old man has been arrested in connection with the attack at Manchester Arena last night.
Theresa May says police think they know identity of lone bomber as she contrasts "warped" terrorist with courageous emergency services.
Donald Trump has branded those who take part in terrorist acts as "evil losers" in the wake of a suicide attack at Manchester Arena.