A bomb exploded in a prison in the southern Yemeni province of Dalea today, injuring four inmates and allowing around 40 prisoners to escape, a government official said.
Witnesses and southern media said all those who fled the police jail belonged to Yemen's southern secessionist movement, which opposes the rule of President Ali Abdullah Saleh.
Recent months have seen increasingly violent clashes between separatists and security forces, and analysts say impoverished Yemen could face a sustained insurgency from southerners unless the government seriously addresses their grievances.
North and South Yemen united in 1990, but many in the south - home to most of Yemen's oil industry - complain northerners have seized resources and discriminate against them.
Elsewhere in Yemen's south, an activist was shot dead and three others were injured when security forces dispersed a protest in the city of Radfan in Lahej province.
Western countries and neighbouring Saudi Arabia, the world's biggest oil exporter, fear al-Qaeda is exploiting instability in Yemen to launch attacks in the region and beyond.
Reuters