Human rights advocates are pressing Mauritanian authorities to release activist Biram Dah Abeid as they mark Tuesday's UN International Day for the Abolition of Slavery.

Abeid and his colleagues from Mauritania's Resurgence of the Abolitionist Movement were arrested on November 11 after they organised a campaign to highlight ongoing practices of slavery in the country. The activists were charged with inciting racial hatred under Mauritania's terrorism laws, according to their lawyer.

In June, Abeid challenged President Mohamed Ould Abdel Aziz in the presidential election, winning nine per cent of the vote. He contested the legitimacy of the results.

There are more than 20,000 signatures across several online petitions calling on the Mauritanian government to free Abeid and the other activists.