Bereaved families of COVID-19 victims have begun painting a mural featuring nearly 150,000 red hearts - one for each death - to remember their loved ones.
It is expected to stretch more than a kilometre long when finished along the southern bank of the River Thames outside St Thomas' hospital, in central London, where Boris Johnson was treated last year.
Visiting the mural on Monday, the Labour leader praised the "remarkable memorial".
Sir Keir Starmer said: "It's very moving and emotional to hear first-hand from those who have lost someone and what it means to them.
"You can see the emotion in them having to tell us again what I know they've told many, many times.
"The hardest bit of our job is... talking to the families about the experience they have been through."
Organisers said that, despite being opposite the Houses of Parliament, the memorial wall was not meant to be "political or antagonistic" but provide a "visual representation" of every life lost.
It has been co-ordinated by the campaign group COVID-19 Bereaved Families for Justice, who have previously called for an inquiry into the government's handling of the pandemic.
During his visit, Sir Keir spoke to Matt Fowler, 33, co-founder of the campaign group, who lost his 56-year-old father Ian to the virus.
"Each heart is individually hand-painted (and) utterly unique, just like the loved ones we've lost," Mr Fowler said.
"And like the scale of our collective loss, this memorial is going to be enormous."
A spokesman for the campaign said permission had not been obtained for the mural, but that Lambeth Council had been informed, and those taking part had offered to clean the area if told to do so.
"We're hopeful that with support from the council and other political quarters it will become an official and more permanent memorial site," they said.
"Boris Johnson said recently people should be able to reflect, one year on, in whatever way they thought appropriate and this is our way."
Sir Keir called for the prime minister to engage with the families of the deceased and to visit the mural in person, which he said was "very important".
"The prime minister should meet the families...and not just meet, listen," he said.
"The most powerful thing about meeting the families is hearing what they have to say.
"I think we owe it to every single family to hear them and to hear them properly.
"There is a huge number of people who have died and each one of these hearts represents a grieving family.
"I think this memorial is a very fitting way, but it is only one way of remembering these families and what they have been through."
Work on the mural is expected to last several days and the group has invited families through local networks to come and take part.
Mr Fowler added: "We know not everyone can come down here to see it, but we really hope this can become a focal point for remembering this national tragedy.
"The objective of this here today is to memorialise and to memorialise with dignity.
"It's not just about me remembering my dad, although of course I am - the first heart I drew on the wall this morning was for him.
"Everyone that died was a real person with a family and friends that miss them and are trying to deal with their grief and their loss."
A Lambeth Council spokesperson said: "In Lambeth, more than 500 people have sadly lost their lives to COVID-19 over the past 12 months, and it is important we reflect on these losses and what they mean to the loved ones and communities they leave behind."