Sir Tony is being inducted into the Order of the Garter as a "knight and lady companion".
Buckingham Palace describes the purpose of the award as to "honour those who have undertaken public service, who have contributed in a particular way to national life".
This move has sparked anger, with a petition calling for the rescindment of Sir Tony's knighthood recently receiving more than 1.5 million signatures.
Much of the criticism centres around his role in the Iraq War.
Part of the petition states: “Tony Blair caused irreparable damage to both the constitution of the United Kingdom and to the very fabric of the nation's society.
"He was personally responsible for causing the death of countless innocent, civilian lives and servicemen in various conflicts. For this alone he should be held accountable for war crimes.”
These sentiments are shared by the group organising today’s protest, the Stop the War coalition.
The organisation was responsible for the largest ever protest in the UK, opposing the Iraq War, and has recently campaigned for the Government to work towards peace in Ukraine.
Chris Nineham, Vice Chair of the group, stated: “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.”
The slogan of the protest is “Jail Blair! No knighthoods for war criminals”.
Signs displayed dub him ‘Bliar’ and protesters have been posing as the former PM.
They have been wearing paper masks of his face and painting their hands red in condemnation of his decision to go to war in Iraq.
One wore a caricatured mask of Sir Tony, complete with red paint on his hands and handcuffs to represent the protest’s depiction of him as a war criminal.
Calls to free Julian Assange, founder of WikiLeaks, and stop his potential extradition to the US were also widespread.
Support for Palestine was visible, with the country's flag being displayed in the crowd.
Sir Tony Blair
Ben Stansall
Sir Tony Blair during today's ceremony
Ben Stansall
