A minute's silence has been held to mark one week since the Westminster attack on 22 March, in which four victims and the attacker died.
Hundreds of people gathered on Westminster Bridge in London to pay their respects.
They included police officers, community organisations and religious groups.
Similar events are being held in Sheffield, Birmingham, Leeds, Leicester and Manchester.
The attacker, 52-year-old Khalid Masood, killed three people when he drove his car into pedestrians walking along Westminster Bridge.
The victims have been named as US tourist Kurt Cochran, sixth-form college employee Aysha Frade, and retired window cleaner Leslie Rhodes.
After crashing, Masood then attacked and killed PC Keith Palmer outside the Houses of Parliament, before being shot dead by police.
Fifty people were also injured.