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Attached to the floral tribute was a note reading: "In memory of those who were lost and in defiance of those who would seek to silence our democracy."
May laid the wreath in Parliament Square, just yards from the scene of the attack by Islamist Khalid Masood, 52, who was shot dead in the attack.
It was the first of five attacks on Britain last year which police blamed on terrorism.
The dead police officer was PC Keith Palmer, 48. Of the other casualties, two were British, one Romanian and one American. Some 50 people were injured.