Witnesses told AFP that the attack on demonstrators in the square in front of Sanaa University was carried out by men wearing masks, whom anti government activists said were "thugs" loyal to the regime.

The attackers used guns, clubs and daggers.

Thousands of people have camped out in the square since February 21, demanding the departure of President Ali Abdullah Saleh, who has been in power since 1978.

Witnesses said that dozens of people were injured when police and loyalists of the ruling General People's Congress party attacked protesters in the square with live rounds and tear gas on Sunday.