Each volunteer held a photograph of one of the 22 Syrian volunteers who lost their lives while trying to deliver humanitarian aid as the charity marked its 150th anniversary.
It was part of a plea for an end to the conflict and to ensure the safety of aid workers across the world.
The charity's chief executive Sir Nick Young said: "We are standing in solidarity with our friends and colleagues in the Syrian Arab Red Crescent to say enough is enough.
"Across Syria, volunteers are risking their lives to bring vital aid to people in desperate need.
"Intentional targeting of aid workers, who play no part in the conflict and who seek to help those in need, is unacceptable. This must stop. The movement is asking all parties to the conflict to respect humanitarian workers and allow them to work in safety."
The charity movement started in 1863, inspired by Swiss businessman Henry Dunant. He had been appalled at the suffering of thousands of men who were left to die due to lack of care after the Battle of Solferino in Italy in 1859.
Dunant proposed the creation of national relief societies, made up of volunteers, to provide neutral and impartial help to relieve suffering in times of war.