The Huthi militants freed on Saturday around 500 prisoners from a main prison in Yemen's southern Ad Dali city.
According to witnesses, forces loyal to former President Ali Abdullah Saleh had handed over the prison to the Houthis, who later set the prisoners free.
Yemen has remained in turmoil since last September, when the Huthis overran the capital Sana'a, from which they have since sought to extend their influence to other parts of the country.