Teddy bears are to be piled outside the gates of Downing Street in a poignant message from campaigners about the human cost of the Syrian conflict.
Celebrities including The Great Gatsby star Carey Mulligan will join protesters on Whitehall, central London, to demand the Government intervene over Russian and Syrian bombing campaigns.
A group of children will lay 200 bears - reflecting the number of young casualties in Aleppo after the ceasefire collapsed last month - at the foot of the street.
They will then deliver a letter to Number 10 demanding Theresa May take action to help cease the bloodshed in the war-ravaged nation.
It comes after the Prime Minister insisted on Friday the EU must keep "all options" open if Russia continues to commit atrocities in the conflict zone.
Campaigners said the day of action is a rebuttal to Foreign Secretary Boris Johnson's call for protests outside the Russian embassy - as they feel the British Government needs to play a stronger role itself.
Among the organisations involved are Amnesty International UK, Avaaz and Christian Aid, and Labour MP Alison McGovern is also expected to talk.
A spokesman for Avaaz said: "What we are witnessing in Syria is one of the greatest humanitarian crises of the 21st century.
"The slaughter in Aleppo is unfolding under our noses but the world is not responding. We cannot claim we do not know.
"Diplomacy has so far failed. The UN is being ignored.
"The UK must not sit on the sidelines any longer.
"The Prime Minister must take robust action to help protect the children of Aleppo."