"Set Them Free" declared a banner on the red double-decker bus, which was adorned with photos of the campaigners friends and relatives they say are currently detained by the Syrian regime.
The Families for Freedom campaigners say they will tour across Europe to gather support for their cause, and hope to drive the bus all the way to Damascus.
Noura Ghazi Safadi, the wife of the prominent Syrian activist Bassel Khartabil, who was reportedly executed in a Syrian government prison, was among the women in the campaign.
Noura, who was a successful lawyer and is the daughter of a political prisoner, met the award-winning open-source software developer and leading pro-free speech activist at a protest in Syria in 2011.
In 2012, he was detained by the Assad regime and taken to the notorious Adra Prison in Damascus, where Noura could still visit him several times a week.
From there, he was transferred to an unknown location, with Noura waiting for news for years – only to be told in August this year he had been killed, according to an interview with The Sun.
As part of their tour in London, Families for Freedom, which is a women-led campaign for the rights of Syria's disappeared and detained, sang the chimes of the iconic Big Ben from the roof of their bus.
Over the next four years, Big Ben will miss over 200,000 of its bongs – the same number as are estimated to be detained in Syria today, the activists said.
| Over the next four years, Big Ben will miss over 200,000 of its bongs – the same number as are estimated to be detained in Syria today |
Hundreds of thousands of ordinary Syrians are detained or disappeared, the majority of them at the hands of the , the campaign's .
"Our position is against enforced disappearance and arbitrary detention by the Syrian regime and all parties to the conflict. We want to mobilise the public to pressure all sides to comply with our demands.
"We, as families, demand the immediate release of our relatives who have been unlawfully detained."
Credit for images: Getty Pictures