"Love is stronger than hate. Light is stronger than darkness. Life is stronger than death," he said. "It was true a year ago. It is as true today."

After the service, families of the dead planned to plant an olive tree on the cathedral grounds before a national minute of silence at 4.30pm.

In a statement, the Prime Minister paid tribute to the bravery of first responders and others, including Ignacio Echeverria, a Spanish man who tackled the attackers with his skateboard and was killed.

Australian nurse Kirsty Boden was among those killed in the attack.

May said the fact that seven of the eight victims came from outside Britain – from France, Spain, Australia and Canada – was "a reflection of our great cosmopolitan capital, whose energy and values brings together people from across the world, and a tragic reminder that the threat from terrorism transcends borders and impacts us all."

Loading

"Our resolve to stand firm and overcome this threat together has never been stronger," she said.

The London Bridge carnage was one of a string of attacks in Britain in 2017 involving Islamic or far-right extremists that killed 36 people in all.

Britain's official threat level from terrorism currently stands at "severe," the second-highest of five levels, meaning that an attack is highly likely.

"We expect the threat from Islamist terrorism to remain at its current, heightened level for at least the next two years, and that it may increase further," the government said on Sunday. It said the threat from extreme-right violence is growing.

Home Secretary Sajid Javid said he planned to recruit 2000 new security service officers to help combat the threat.

AAP