Anti-racism protesters, who have taken to the streets following the death of African American George Floyd, have put statues at the forefront of their challenge to Britain's imperialist past.
A statue of Edward Colston, who made a fortune in the 17th century from the slave trade, was torn down in the city of Bristol last Sunday, and authorities have acted to protect monuments they believe could be next.
They have now boarded up a statue opposite parliament of Churchill after demonstrators daubed it with paint last weekend.
"It is absurd and shameful that this national monument should today be at risk of attack by violent protesters," Johnson wrote on Twitter.
On Friday, around 500 people gathered in Hyde Park chanting "the UK is not innocent" and "Black Lives Matter", before marching through central London, with many saying that statues such as Colston's were legitimate targets.
"If we have these big images, and we're telling people that these people and what they stood for is OK, we're just allowing everything that they did to pass," said student Samantha Halsall.
Organisers urged protesters not to turn up in central London on Saturday amid concern that there could be clashes with counter-protestors planning to defend statues.
Police imposed route restrictions on both groups and said demos must end by 5 pm. (1600 GMT).
Johnson is an admirer and biographer of Churchill, and some of those close to him say he wants to emulate him.
But Churchill expressed racist and anti-Semitic views and critics blame him for denying food to India during the 1943 famine which killed more than two million people - aspects of his legacy which some say are not scrutinised enough.
"Yes, he sometimes expressed opinions that were and are unacceptable to us today, but he was a hero, and he fully deserves his memorial," Johnson wrote.
"We cannot now try to edit or censor our past. We cannot pretend to have a different history."