Yemeni Zaidi rebels demanded dismantling the national security services at a Thursday funeral in Sanaa of 13 fellow Shiites killed in weekend clashes with police, as Human Rights Watch demanded the incident be probed.
"The people want to dismantle the national security services," chanted thousands who marched from University Square in the north towards the Grand Mosque in central Sanaa, before burying the dead whose bodies had been carried aboard vehicles in the procession.
A statement from the rebels, who have re-named themselves Ansarullah (Supporters of God), said 13 people were killed and 100 were wounded in the Sunday clashes. A security official had told AFP 10 rebels died.
On Monday, the Zaidis staged a similar protest and urged the government to address the killings.
A gunfight erupted on Sunday when rebels, among a crowd demanding the release of members of their community, attacked the security headquarters with automatic weapons and grenades, an official said.
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They were said to have been well prepared and used neighbouring buildings to launch their raid, adding that those killed and wounded were among the attackers.
The official said earlier that some of those detained were "suspected of (sharing) intelligence with Iran," a predominantly Shiite country that Sanaa has previously accused of backing the rebels.
On Wednesday, Human Rights Watch urged the government to "ensure that its promised investigation into the incident is carried out promptly, impartially and thoroughly, and results in appropriate prosecutions of those responsible for serious abuses."
It accused authorities of using "lethal force against an apparently peaceful demonstration."
The rebels, also known as Huthis after their late leader Abdel Malek al-Huthi, rebelled in 2004 against the government of ex-president Ali Abdullah Saleh, accusing it of marginalising them politically and economically.
Fighting between them and Yemeni forces killed thousands of people before a ceasefire was reached in February 2010.
The Zaidis are participating in an ongoing national dialogue launched in March to discuss the impoverished country's main problems, including the issue of the Huthi rebellion.