London (AFP) - Tens of thousands of people marched through London in protest against Britain's refugee policy Saturday, with new Labour Party leader Jeremy Corbyn attending in his first political engagement only hours after being elected.

The march to Parliament Square, which went past Prime Minister David Cameron's Downing Street office, urged solidarity with the huge numbers entering continental Europe.

Marchers on the peaceful rally danced to music, blew whistles and waved placards reading: "Refugees welcome here", while others read: "No human being is illegal", "Open the borders", and "Refugee lives matter".

Britain, the biggest donor of aid to Syrian refugees after the United States, has agreed to take in 20,000 refugees over five years -- far fewer than other major European countries.

They will specifically be drawn from those in United Nations refugee camps on Syria's border, rather than those with the means to reach continental Europe.

"The people who are there are often more vulnerable than those who get out," said marcher Pauline Aaron, who said she had worked in refugee camps in Africa and southeast Asia.

"But that crisis is very different from the second humanitarian crisis in Europe."

"I want to support the refugees," said Deborah Flatley, who held a home-made cardboard sign reading: "We admire your bravery. You deserve a safe and happy life. We welcome you here with open arms".

Another marcher, Dusan Petkovic, told AFP: "This rally is making clear that the government is wrong in their stance towards refugees."

Britain is not participating in a European Union quota system to share out refugees who have already reached Europe.

Petkovic dismissed the 20,000 figure as "pathetic" with more than four million Syrians fleeing the four-year civil war.

Another demonstrator, Marc Faux, said Britain's response, over the next five years, was "ignoring the problem we have right now".

- 'I'll be your friend' -

At the rally, a boy dressed as Paddington Bear had a sign saying: "Paddington Bear Was A Refugee".

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A little girl held up her drawing of two hands clasped together with the words: "Help Syria" and "I'll be your friend".

One man chalked "No more deaths in the sea" on the ground.

The march was organised by a group of non-governmental organisations including the left-wing Stop the War Coalition, which was co-founded by Corbyn.

The new Labour leader was greeted with huge cheers when he addressed the crowds from the back of a truck in Parliament Square.

"Recognise your obligations in law," he said, addressing the government.

"But above all, open your hearts and open your minds ... towards supporting people who are desperate, who need somewhere safe to live, want to contribute to our society, and are human beings just like all of us.

"Together in peace, together in justice, together in humanity, that surely must be our way forward."

His speech was concluded with a rendition of the socialist anthem "The Red Flag".

Some of those on the march were attending their first rally.

"The image of the boy on the shore really got to me. That was the tipping point," said Baljeet, 23, a marketing intern from Canada.

"People are escaping war and we're not letting them in," she told AFP.

She added: "I was appalled at the racism. Hungary is building fences -- while in Germany they're handing out teddy bears."