A group of intellectuals and politicians has called on France to repay 17bn euros (£14bn) "extorted" from Haiti in the 19th Century.
The money, they say, would help impoverished Haiti rebuild after January's devastating earthquake, which killed more than 250,000 people.
In 1825 France demanded 150m gold francs in compensation after Haiti gained independence in a slave revolt.
Campaigners say that demand was illegitimate and illegal.
"The 'independence debt', which is today valued at over 17bn euros illegitimately forced a people who had won their independence in a successful slave revolt, to pay again for the freedom," said the appeal to President Nicolas Sarkozy, published in the French newspaper Liberation.
The signatories included American linguist Noam Chomsky, French philosopher Etienne Balibar, and the Euro MPs Daniel Cohn-Bendit and Eva Joly.
Haiti gained independence from France in 1804, but French slave owners demanded compensation for lost property.
The figure of 150m francs was reduced to 90m, but still not paid off until 1947.
The authors of the letter said the case for repayment of the debt was "morally, economically, and legally unassailable".
"In light of the urgent financial need in the country in the wake of the devastating earthquake of January 12, 2010, we urge you to pay Haiti, the world's first black republic, the restitution it is due," the letter said.
International donors in March pledged £3.4bn in aid to Haiti - but so far only a fraction of that has been paid.
In February President Sarkozy toured Haiti and pledged 270m euros (£235m) in reconstruction aid. That included cancelling 56m euros (£48m) in Haiti's modern debt.