Supporters said they were planning to stage songs, speeches and readings in several European cities.
Assange, 44, is wanted for questioning over a 2010 rape allegation in Sweden but has been inside Ecuador's UK mission for four full years in a bid to avoid extradition.
The anti-secrecy campaigner, who denies the allegation, walked into the embassy of his own free will on June 18, 2012, with Britain on the brink of sending him to Stockholm, and has not left since.
The Australian former computer hacker fears that from Sweden he could be extradited to the United States over WikiLeaks' release of 500,000 secret military files, where he could face a long prison sentence.
Listed participants in Sunday's anniversary events include Patti Smith, Brian Eno, PJ Harvey, Noam Chomsky, Yanis Varoufakis, Ai Weiwei, Vivienne Westwood, Michael Moore and Ken Loach.