The "Chubby hearts over London" campaign is a designed to be a love letter to London.
The balloons have been suspended over, and sometimes squashed within, 29 locations including Battersea Power Station, Covent Garden, Leciester Square, Somerset House, Trafalgar Square and Wellington Arch.
The hearts will also takeover Piccadilly Lights for a 40-second slot during the early evening on Valentine’s Day. It will also feature on four other Ocean Outdoor sites.
The campaign begins today and is running throughout London Fashion Week.
Hindmarch said: "The idea for this project came to me whilst sitting in a packed Royal Festival Hall. I was in awe of the resilience of Londoners and its visitors.
"Tying giant helium filled chubby hearts on to buildings all over the city is simply a ‘surprise love letter to London’ as a tribute to its strength and to celebrate our amazing creative city."