The WikiLeaks founder is spending his fiftieth birthday in a UK prison, just as the UK court is considering the US request for Assange's extradition.
The activists said they wanted to send their best wishes to Assange and draw the parliament's attention to the problem.
On 4 January, the UK court declined the US request for extradition, but the United States appealed that ruling. Assange's legal team says the hearings may take years.
The US Justice Department charged Assange with espionage and computer intrusions over WikiLeaks' publishing thousands of classified documents. If extradited and convicted in the US, Assange could face decades in prison.