Prisoners 'injured'

Local sources confirmed that four prisoners were injured in this explosion.

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Also on Thursday, police fired tear gas to disperse protesters in Dali and several other southern towns.

The demonstrators were protesting the government's ongoing crackdown against southern pro-secession activists.

Police officials said scores of protesters were detained.

Tensions have recently escalated in the south, adding to the challenge Yemen's government already faces from an al-Qaeda movement that has set up operations there and a northern rebellion.

Widespread demonstrations have taken place in Yemen in recent months,with crowds calling for the military to withdraw from southern cities and for the government to halt a sweeping campaign of arrests.

South Yemen was independent from 1967 until it united with the north in 1990.

An attempt to break away again in 1994 sparked a short-lived civil war that ended when the south was overrun by northern troops.