London's Metropolitan Police said its officers carried out the arrest at 6 a.m. local time (BST), alongside Welsh counter-terror police officers and members of South Wales Police.
Police have made a seventh arrest in connection with the Parsons Green Tube bomb attack, Scotland Yard has said. He is the fourth person to be arrested in Wales, after three men were detained in the city of Newport last week.
A bomb partially exploded on a train at the Tube station in south-west London during rush hour, injuring 30 people.
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The suspect is now at a police station in south London and is being held under section 41 of the Terrorism Act.
He has been taken to a south London police station where he is being quizzed by detectives.
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Meanwhile, three further men - aged 17, 21, and 48 - have all been released with no further action. Searches are still ongoing at one address in Surrey, south of London, and in Cardiff.
Hassan is understood to have expressed anger towards Tony Blair and have entered the country illegally in 2015 and put into foster care. The home-made bomb engulfed a carriage in flames, but apparently failed to fully explode.
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