The Duke and Duchess of Cambridge and Prince Harry have arrived at a “service of hope”, following last month’s Westminster attack.
Khalid Masood killed three people when he drove into crowds on Westminster Bridge before fatally stabbing a police officer at the Houses of Parliament.
He died after being shot dead by police during the attack on 22 March.
Emergency services staff and relatives of the victims also join the congregation at Westminster Abbey.
About 2,000 people are expected to attend the multi-faith service, including members of all major denominations and faiths in the UK.
The Dean of Westminster, the Very Reverend John Hall, said the Royal Family were a “focus of unity” for the nation and they would be meeting the families of the victims after the service.
BBC correspondent Peter Hunt said the service would attempt to offer hope rather than being a memorial.
There will be a focus on the diverse nature of society, with prayers offered to protect the country from the forces of division and hatred, he said.
The service, which is being broadcast on BBC One, will include a Muslim prayer from London Mayor Sadiq Khan.
The victims were Aysha Frade, 44, who worked at a London sixth-form college; American tourist Kurt Cochran, 54, from Utah; retired window cleaner Leslie Rhodes, 75, from south London; and father-of-two PC Keith Palmer, 48.
It will be the first time the families of all the victims have come together in the same place.
Home Secretary Amber Rudd, Commons Leader David Lidington and Met Police Acting Commissioner Craig Mackey have arrived at the service.
Attendance is by invitation only.
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