Concrete blocks have been placed outside Westminster, as part of heightened security measures around London.
The Metropolitan police said the improved security was not in response to a specific threat, but was meant to ensure the security of Londoners and people visiting the city.
"These measures are being carried out on a precautionary basis in light of events around the world and the fact that security in the capital remains at a high level."
These "events around the world" include suicide attacks in Saudi Arabia and Morocco, raising fears Al-Queda may have regrouped.
Along with concrete blocks outside Westminster, which may be intended to stop a suicide attack, anti-terrorist measures will be stepped up at Heathrow Airport.
Tanks and troops were stationed outside the airport for several days in February.
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