A police officer can be seen in the live video feed from a camera worn by one of the climbers at the top of The Shard. It is not clear whether any of the protesters have been arrested.
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The singer had been tweeting in support of migrants when she was bombarded with abusive messages.
Ministers are reportedly considering plans in a bid to lower inward migration.
In a tweet on Saturday, the US president didn't give a reason for his absence but said he wished"everyone well and have a great evening".