The British actress, who is a global ambassador for War Child UK, joined charities Amnesty International UK, Doctors of the World UK and Syria Relief to put pressure on Theresa May’s government to act to end the suffering.
Large crowds gathered in Whitehall, holding placards which read “Save Aleppo” and “No bomb zone now”, while others flew Syrian flags.
Mulligan spoke to reporters while holding a teddy bear which belonged to her one-year-old daughter.
Mulligan 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.”
The 31-year-old was one of hundreds to lay a teddy bear at the gates of Downing Street.
The bears represented the 191 children who have been killed in the city since the attacks started a month ago.
About six children are killed in the city every day.
Since the recent breakdown of a fragile cease-fire, Aleppo has been the focus of continued aerial strikes.
Schools and hospitals have been systematically targeted, and civilians’ water and electricity deliberately cut off, War Child said.
The charity is calling for a stronger response from the international community to the atrocities being committed in Aleppo and elsewhere in Syria.
War Child said that repeated calls for a ceasefire are not working and are calling for a clear signal to be sent to those who commit these atrocities that there will be consequences for their actions.
Mulligan said: “War Child’s message to our government is this - Not only is Aleppo a tragedy, it is a line in the sand.
“And we’re here to call on the UK to grasp this opportunity to lead from the front. The UK can play such an important role in bolstering the international community’s resolve to finally put in place the robust measures we need to end the fighting.
“We’re calling on you both, Theresa May and Boris Johnson and we have faith in you to act now, and help Save Aleppo’s children.”
War Child is demanding that the British government develops a robust plan to end the fighting and ensure that international humanitarian law is upheld.
This includes measure such as economic sanctions, international justice, implementing no fly zones and no bombing zones.