Ahead of fresh marches planned later today by the far-right populist "Patriotic Europeans against the Islamisation of the Occident" (PEGIDA) group, Merkel said a right to demonstrate did not extend to "rabble-rousing and defamation" against foreigners.
She told reporters that those taking part in the protests "should take care not to be exploited" by radical elements trying to harness fears of rising numbers of foreigners in Germany to drive an extremist movement.
A poll for news website Zeit Online showed that nearly one in two Germans -- 49 per cent -- sympathised with PEGIDA's stated concerns and 30 per cent indicated they "fully" backed the protests' aims.
Almost three in four -- 73 per cent -- said they worried that "radical Islam" was gaining ground and 59 per cent said Germany accepted too many asylum seekers.
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Justice Minister Heiko Maas told the daily Sueddeutsche Zeitung that the marches "bring shame" on the country and that Germans should stand up to racism.
He warned that Germany, amid a record influx of asylum seekers from countries stricken by war and poverty, is experiencing an "escalation of agitation against immigrants and refugees" and called the trend "repugnant and abhorrent".
Since the protests have grown in size, a debate about immigration and refugees has gripped Germany, a country whose Nazi past makes expressions of xenophobia especially troubling.