Witnesses said some others were arrested, but officials said they were released within hours.

In Sanaa's Change Square, Yemeni security forces shot live ammunition at protesters, injuring 11, witnesses and field medical teams said.

All 11 had gunshot wounds, witnesses said. Two of the injured are in serious condition, medical staff at the square said.

"We were being shot at directly," said protester Abdullah Kulaibi. "Security forces were trying to kill us and not disperse us."

Yemeni officials have repeatedly denied accusations of targeting peaceful protesters.

Witnesses said at least 20 protesters were arrested on April 17. A security official said they were released about two hours later. The official asked not to be identified, as he is not authorized to speak to the media. Field medical teams in Sanaa said some of the releases came because the detainees needed medical treatment after inhaling tear gas. One medical team member, Osama Qurashi, said those who were seriously impaired were taken to nearby hospitals, while the rest were taken to a field medical hospital set up in Change Square.

For weeks, demonstrators have taken to the streets calling for President Ali Abdullah Saleh to step down.