Hundreds of people gathered in Whitehall, central London, in a rally calling for the UK Government to take decisive action to end the bloodshed in the war-ravaged country.
The crowd included children wearing “Save Aleppo” t-shirts and other people carrying placards urging a “No bomb zone now”, while some flew Syrian flags.
Mulligan joined the demonstration just opposite the gates to Downing Street, where teddy bears were later laid in a poignant message from campaigners about the human cost of the long-running conflict.
A small light brown-coloured teddy belonging to The Great Gatsby star’s one-year-old daughter Evelyn was among the pile, and the actress told how becoming a parent has motivated her further to raise awareness and try to help.
She told the Press Association: “I brought one of my daughter’s teddy bears here today and ever since having my child - I’ve worked with War Child for a couple of years now - but since having my daughter it just drives home even more how unimaginable it would be for my daughter to be in any of these situations and to have to deal with any of this.
“It just really drives me to speak out and do more if I can.”
She described the demonstration as the opportunity to “stand up and say that we need to do something real”.
Mulligan, an ambassador for War Child, has previously spoken out to say the inaction in Calais where many unaccompanied refugee children became stranded in the so-called Jungle camp, made her ashamed to be British.
Regarding the arrival of young refugees in Croydon in recent days, she said: “I feel very proud to be British in this regard. We have made a strong stance this week.”
She added: “I think we really can lead the way here and we can lead people and work with our international allies to come up with a really robust plan to finally put this to an end.”
She recalled the night-time ritual of putting her daughter to bed and told the crowd gathered: “I’m safe in the knowledge that when I put Evie down to bed she is safe.
She added: “The parents in Aleppo aren’t. They don’t know what the night will bring.”
A letter has been delivered to Number 10 demanding Theresa May take action to help end the violence.
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.
Labour MP Alison McGovern’s voice broke with emotion as she remembered her friend the late Jo Cox MP’s work in helping civilians in Syria.
She told the rally: “It shouldn’t be me who is speaking here today, it should be my friend Jo Cox, but because she isn’t here, we have to be here.
“Fifty thousand children have been killed in Syria since 2011, and millions have fled their homes.
“There are 100,000 children trapped right now in Aleppo.”
Among the other organisations involved in the campaign are Amnesty International UK, Avaaz and Christian Aid.
Bert Wander, Avaaz campaign director, said: “The bloodbath in Aleppo has gone on long enough, and Theresa May must listen to the people on the streets and support a no-fly zone to stop the slaughter.
“How many schools, hospitals and families must be bombed before Syria becomes a priority? If the prime minister fails to act, history will judge her harshly. “