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A man has been arrested in relation to Friday's bomb attack in London, which left 30 people treated in hospital.

Police in Dover - on the UK's south coast - detained an 18-year-old man who is due to be transferred to custody in London.

"This arrest will lead to more activity from our officers," they said in a statement, outlining that the teenager was being held under anti-terror legislation.

The bomb failed to detonate fully, according to media reports, but flash burns on passengers led to some injuries.

The Islamic State group claimed responsibility for the attack

Prime Minister Theresa May announced late Friday that troops - 1,000 were deployed - would take on responsibility for guarding key sites, including nuclear facilities, to free up police.

In a statement, anti-terrorism chief Mark Rowley said officers were "chasing down suspects".

"Somebody has planted this improvised explosive device on the Tube. We have to be open-minded at this stage about him and potential associates," he said.

The critical alert terror warning was last used after the deadly suicide attack at Manchester Arena, also claimed by IS, in May.