Britons are taking part in a “guerrilla campaign for kindness, tolerance and understanding” using the hashtag #thisisthoughtful.
“I am a dual American and British citizen,” she writes on the campaign’s website.
“Over the past year I’ve shed tears watching all my home countries, to put it lightly, lose it.”
She continues: “This planet is home to all of us, and it’s up to each of us to take responsibility, and action, for keeping it together.”
People have been using the place cards to spread their own message of unity in the hopes of making readers feel welcome and at home in London.