Assange is wanted by U.S. authorities on 18 counts, including a spying charge, relating to WikiLeaks' release of vast troves of confidential U.S. military records and diplomatic cables which Washington said had put lives in danger.
The 96-year-old, who has been forced to cancel a series of engagements recently due to "episodic mobility problem," reluctantly pulled out of Friday's service after experiencing "some discomfort" the previous day.
Tens of thousands of royal supporters waving flags lined the streets of London for a military parade that marked the start of Platinum Jubilee events. Millions of people across Britain and the world were expected to join street parties, watch the festivities and light beacons in honor of the 96-year-old queen.
With the queen forced to withdraw for the first time in almost 60 years, Charles stepped in to read out the government's legislative agenda at the Palace of Westminster, the first time he has taken on such a major constitutional duty.