London police say a second man has been arrested in connection with the London subway attack.

Police said today a 21-year-old man was arrested late Saturday night in Hounslow in west London.

He was arrested under the Terrorism Act.

Two men are now in custody for possible role in the attack that injured 29 people.

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Britain's terror threat level remains at "critical" - the highest level - meaning that authorities believe another attack is imminent.

Police last night arrested an 18-year-old man in the port of Dover - the main ferry link to France - and then launched a massive armed search in the southwestern London suburb of Sunbury.

- AP