Brussels (AFP) - Up to 20,000 people demonstrated in the Belgian capital Sunday in support of refugees, demanding they be welcomed and treated properly.

The organisers, a collection of local and international rights groups, said there were "20,000 people in Brussels to say welcome to refugees."

"Thanks for this fantastic effort," the 'Citizens Platform for support of refugees in Brussels' said on Facebook.

"Standing behind us are all the European citizens who are working to help the refugees," Elodie Francart of the Citizens' Platform group was quoted as saying by the Belga news agency.

The marchers carried banners reading "Refugees Welcome" while many tweeted messages of support and encouragement.

Police said some 15,000 people took part, gathering near the Gare du Nord train station close to what is known as the "Foreigners Office" where migrants must submit their papers and applications for asylum.

In a nearby park, local people have built a tent village in recent weeks to offer food, clothing and shelter for the many migrants fleeing conflict and upheaval in Iraq, Syria, Somalia, Eritrea and Afghanistan.

Belgium has a large immigrant community and the recent influx has largely been tolerated but the centre-right government has faced criticism that it has not done enough.

The authorities say they are doing everything possible given the numbers involved but insist the rules on asylum must be followed.