NIAMEY - Unidentified gunmen opened fire on a prison in the centre of Niger's capital Niamey on Saturday, a week after al-Qaida-linked fighters launched twin attacks on a military base and a French uranium mine, witnesses said."We were sitting there when we saw these armed men start to shoot at the guards from the civilian prison. I saw several of them fall and not get back up," Ila Yaye, who lives near the prison, told Reuters.