Several thousand people could be seen protesting at the march through central London to Prime Minister David Cameron's office, brandishing placards reading: "Open the Borders" and "Refugees In, Tories Out".
"This rally is making clear that the government is wrong in their stance towards refugees," one of the protesters, Dusan Petkovic, told AFP.
Read: Cameron confirms UK staying out of EU refugee quotas
Cameron had a belated change of heart on letting in more Syrian refugees as the migrants crisis in Europe escalated and last week agreed to take in 20,000 over five years.
The premier said they would be taken directly from UN refugee camps and the British government has refused to take part in a European Union quota system to share out refugees who have already reached Europe.
Read: Denmark rejects EU refugee quotas
Petkovic dismissed the 20,000 number as "pathetic".
Another protester, Marc Faux, said: "The government's response is ignoring the problem we have right now."
Pauline Aaron, who said she had worked in refugee camps in Africa and southeast Asia, referred to the issue of migrants in Europe as "a second humanitarian crisis" after the one in camps around Syria's borders.
Read: Britain offering 4,000 spaces for Syrian refugees - UNHCR
At the protest, a boy dressed as Paddington Bear had a sign saying: "Paddington Bear Was A Refugee".
A little girl held up a drawing of two hands clasped together with the words: "Help Syria, I'll be your friend".
The rally was being organised by a group of non-governmental organisations including the Stop the War movement, which was co-founded by Corbyn.
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