Spanish media says between 3,000 and 4,000 state officers have already arrived, or are on their way to Catalonia. The reinforcements will join 5,000 state police already based in the region.
The homemade bomb engulfed a carriage in flames but apparently failed to fully explode. It was Britain's fifth major attack so far this year.
Germans are heading to the polls on September 24 for a federal election which will decide whether the incumbent chancellor, Angela Merkel, is granted a fourth four-year term in power.
The AfD, which is anti-immigration and anti-Islam, is set to become Germany's first hard-right nationalist party to enter parliament in the post-war era.
Thousands gather at the gates of Catalonia's judicial body in Barcelona to demand the release of a dozen officials arrested in connection with the vote on independence that Spanish authorities are challenging as illegal.
Trump said Washington was weighing if the deal serves its security interests, but Germany and other powers fear its collapse could trigger regional arms race.
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