"The public will see more officers, including armed police, in and around transport hubs," a Scotland Yard spokesman told AFP.
The item, found Thursday on a train at North Greenwich station, which serves the O2 entertainment complex in southeast London, was being forensically examined.
Counter-terror officers attended an address in Newton Abbott in southwest England on Saturday as part of the inquiry and "found an item they deemed suspicious", Scotland Yard said.
The 19-year-old arrested yesterday remains in custody for questioning on suspicion of the commission, preparation and instigation of terrorism acts.
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Armed police arrested him in London in connection with the suspicious package found at North Greenwich station.
An electric stun gun was used during the arrest on a busy shopping street, but no shots were fired.
Britain's current national terror threat level has been set at severe -- the fourth-highest of five -- since August 2014, meaning an attack is considered highly likely.
Scotland Yard refused to comment on the report.
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