The emergency Downing Street demo is being held on the eve of a parliamentary recall confirmed by Prime Minister David Cameron today.
His announcement coincided with a bloodthirsty intervention by ex-PM Tony Blair.
Blair claimed that failure to intervene in Syria could create a "nightmare scenario" for the West in the Middle East.
The ex-PM, now Middle East "peace envoy," claimed in the Times: "I know as one of the architects of policy after 9/11 the controversy, anguish and cost of the decisions taken.
"I understand every impulse to stay clear of the turmoil," he added, before declaring that people had to understand the consequences of "wringing our hands instead of putting them to work."
But Stop the War Coalition convener Lindsey German retorted that as the joint architect of the disastrous war in Iraq and the invasion of Afghanistan Blair was "the last person we should be taking advice from."
She said: "He is totally discredited - even his own party are very sceptical about intervention.
"Ten years ago he told us to stop hand-wringing and where did it get us? Iraq, Afghanistan and Libya.
"He has learnt nothing from his mistakes and seems intent on repeating them - which is Albert Einstein's definition of madness."
She was also scathing about Blair's continually hawkish stance given his role as Middle East peace envoy.
"He obviously didn't read the job description," she said. "He is one of the keenest advocates of war in the Middle East.
"He wants war in Syria, war in Iran, he supports Israel so he wants war in Palestine - what a great peace envoy."
The No Attack on Syria protest will take place at 5pm on Wednesday August 28 outside Downing Street