David Cameron has spoken to his Libyan counterpart Ali Zeidan following his abduction and promised the UK's support to help build a "stable, free, peaceful and prosperous" Libya, the Prime Minister's spokesman said.
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The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.
A man, apparently armed with a small axe and a large stick, was detained close to the event and police cleared the area as a precaution.