The Israeli air force has launched airstrikes against targets in Gaza, according to reports from Israeli newspaper Haaretz.
The paper also tweeted an image of what appears to be an explosion caused by one of the strikes.
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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.