"The authorities on Thursday began releasing the (Zaidi) prisoners," said a source close to the group, indicating that the decision would affect about 1,000 detainees.
A security official, who spoke on condition of anonymity, confirmed the release and added that "President Ali Abdullah Saleh ordered for 400 prisoners to be released in the first stage."
Witnesses in Saada, the northern Shia stronghold, reported that about 150 prisoners had already arrived in the town.
The members from the Zaidi Shia group, also known as Huthis, have been engaged in sporadic fighting with government forces since 2004.
The prisoner release comes just days after a Qatari delegation arrived in Yemen in a bid to consolidate a fragile peace deal reached in February between the Sanaa government and the Huthis.
The truce has largely held, although a car bomb stuck a religious procession in a Shiite bastion in the northern province of Al-Jawf province on November 24, killing 15 people.
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