Germany's foreign minister along with Russian, Ukrainian and French counterparts have agreed to report back to their leaders and then possibly agree on Monday or Tuesday over how to continue talks over the crisis in Ukraine, according to Reuters.
Frank-Walter Steinmeier said the aim remains to bring about a ceasefire in the country.
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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.