Mauritanian President Mohamed Ould Abdel Aziz on Wednesday called for the west African country to root out hate speech as he headed a rally aimed at dampening ethnic tensions.
"The people who are behind this speech are a minority, but we have to put an end to their toxic behaviour for the sake of the future," the president said at the march in the capital Nouakchott.
He also warned he would use a law adopted last year to crack down on "hateful, racist or violent speech".
A conservative desert country with a populaton of 4.4 million, Mauritania is wrestling with tensions between communities of Arab-Berber and sub-Saharan African descent.
Marchers carried banners promoting ethnic cohesion and chanted "no to hate, no to extremism and inciting violence" at the rally, which organisers said was attended by hundreds of thousands of people.
Mauritanians were given the day off with pay on Wednesday for the march, the first led by Aziz since he came to power in a coup in 2008.
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He called for the gathering in response to ongoing combative disputes on social networks, including Whatsapp, between the country's Arab-Berber community and the Haratines, former slaves and their descendants.
Slavery remains deeply entrenched in Mauritania, the last country in the world to abolish the practice in 1981, after light-skinned Berber-Arab Moors enslaved local black populations when they settled there centuries ago.
Aziz said he recognised the country had "social and economic disparities, like everywhere in the world where there are rich and poor," but the "remedy for this lies in education".
The opposition declined an invitation to the rally, saying "this march cannot not be a solution" to the problems justifying it and calling for a national dialogue to find "sustainable solutions".