They turned down a bulk order for bricks that were to be used in a new artwork about political dissidents as part of an exhibition in Melbourne, Australia.
The refusal prompted an outcry on social media, with many offering their own Lego blocks to complete his installation, some using the hashtag #legosforweiwei.
As offers to donate blocks began to pour in, Ai tweeted to The Guardian: “Yes, I will find a way to accept”.
Ai used Lego last year to create portraits of 175 dissident figures who had been jailed or exiled, from Nelson Mandela to Edward Snowden, on the site of the former Alcatraz prison near San Francisco.
The Chinese artist said the company told the museum its bricks could not be used for artworks containing "any political, religious, racist, obscene or defaming statements".
Referring to The Lego Movie's slogan "everything is awesome", Ai wrote on Twitter: "Lego will tell us what to do, or not to do. That is awesome!
"Lego is giving us the definition of what is 'political', and all the big corporations are telling us what to love or hate. That is awesome."
In an Instagram post, he added: "Lego's refusal to sell its product to the artist is an act of censorship and discrimination."
Lego spokesman Roar Rude Trangbaek would not comment directly on the case but said that, as a principle, Lego "respects any individual's right to free, creative expression", albeit adding that the company had a long-standing policy not to directly sell to anyone if it knew that its bricks would be used to make a political statement.
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