A crowd of protesters gathered in Windsor on Monday as Tony Blair was admitted to Britain's oldest order of chivalry, the Order of the Garter.
The Queen appointed the former prime minister a Knight Companion of the Most Noble Order of the Garter – the highest possible ranking.
The knighthood has sparked a significant backlash from anti-war groups.
It comes 15 years after Blair left Downing Street and 19 years since he led Britain into the Iraq war.
The disgraced Duke of York was forced to remain out of sight during the procession after a “family decision” was taken to limit his appearance to a behind-the-scenes lunch and investiture ceremony.
In December, when the announcement was made, more than 1.5 million signatures were gathered on a petition calling for the knighthood to be “rescinded”.
The petition said Blair was the “least deserving person of any public honour” and that he should be “held accountable for war crimes”.
Stop the War activists gathered at the Queen Victoria statue outside Windsor castle on Monday to demonstrate against the appointment.
Stop the War Vice Chair, Chris Nineham, said: "The rehabilitation of Tony Blair since the Iraq war is a scandal. He is widely recognised as a war criminal.
"Our protest will be representing the millions of people who believe the only court Tony Blair should be going near is the Hague."
Asked about the criticism in April Blair told ITV that he felt a "huge amount of sympathy and distress for the people that were affected by those decisions".
He added: "The only thing I ever say to people is that you can disagree with the decision, but it really is not fair to say the decision was taken on the basis of deceit or lies or whatever people say about it."
Founded in 1348 by Edward III, appointments to the Garter are bestowed by the Queen and made without advice from a prime minister. They are awarded for outstanding public service and achievement.
There are currently 21 non-royal companions, out of a maximum of 24 who can be in the order at any one time.
Once appointed, the position remains for life unless a Knight or Lady offends against certain "points of reproach".
Usually appointments are announced on 23 April - St George's Day - but the Queen has the power to announce a new addition at any time.
Each year, Royal Knights and Ladies of the Order of the Garter gather at the chapel in Windsor for a colourful procession and ceremony.
During Monday’s service, the Duchess of Cornwall was installed as a Royal Lady of the Order of the Garter.
Baroness Valerie Amos was also appointed Lady Companion of the Order.
The Labour member of the House of Lords, who was the first Black person to become a cabinet member, will now also be the first black person appointed Lady Companion of the Order.