Hart inspired children to paint and draw on TV shows for nearly 50 years before he retired in 2001.
Morph appeared alongside Hart from 1977 on several of his UK TV programmes, notably Take Hart and Hartbeat.
The event has been organised on social networking website Facebook by a group of friends who wanted to remember Hart.
Laura Kerry, a 31-year-old arts manager from Walthamstow, won a competition for the Best Morph, which was judged by Ms Williams.
She said: "I only spent four hours making him. I heard about it and I was just really moved and felt we had to be part of it."
There were some unusual interpretations of the well-known figure
Ms Williams, who is currently finishing a biography of her father's life, said: "I've been completely bowled over suddenly to discover that there are hundreds of people with whom I share my father and who probably love him almost as much as I do.
"I had no idea what to expect today. We were saying to ourselves as we came here today 'will there be four or 400'? It's absolutely lovely to see so many people here.
"My father would particularly love this because everybody is doing exactly what he always wanted them to do which was make something, just give it a go, make a model or draw a picture.
"He would be absolutely thrilled. He would have thought: 'You're all completely mad' but he would have been very touched."
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