Demonstrations are planned across France in protest at the government's policy of deporting Roma people.
Anti-racism groups opposed to the moves say that more than 30,000 people may rally in Paris alone.
There has been strong international criticism of the deportations, which saw 1,000 Roma (Gypsies) returned to Romania and Bulgaria last month.
However, opinion polls suggest more than half of French people back the government on the issue.
According to official figures, 11,000 Roma were expelled from France last year.
The League of Human Rights, which called for the demonstrations, said it wanted to counteract government "xenophobia" and what it described as the systematic abuse of Roma in France.
At least 138 demonstrations are planned, including a march through the capital due to start at 1400 (1200 GMT).
The rallies are being backed by the opposition Socialist Party and the General Confederation of Labour (CGT), France's second largest trade union confederation.
'Respecting the law'
France began a high-profile campaign of clearing large numbers of illegal Roma camps last month, as part of a security crackdown announced by President Nicolas Sarkozy.
The move was announced after a number of incidents of violence targeting the police.
In mid-July, riots erupted in Grenoble after police shot an alleged armed robber during a shootout.
The next day, dozens of French Roma attacked a police station in the small Loire Valley town of Saint Aignan, after police shot dead a French Roma man who had allegedly not stopped at a police checkpoint.
The mass expulsions have drawn criticism from the Vatican and the UN and President Sarkozy has also faced dissent from within his own cabinet.
Prime Minister Francois Fillon hinted that he disliked the crude links being made between foreigners and crime, while Foreign Minister Bernard Kouchner said he considered resigning over the issue.
Earlier this week, the European Commission criticised France over its expulsions of Roma, saying it did not put enough emphasis on the individual circumstances of those facing expulsion.
Under EU rules, the state can expel people who have been in the country for at least three months without a job or are a social burden. They can also be expelled within three months of their arrival if they are deemed to be a threat to public security.
France said it has been "scrupulously respecting European law", and that most of the repatriations were voluntary.
BBC News
