Unidentified gunmen firing on an anti-government rally in the Yemeni capital Sanaa have killed at least 39 people and injured 200, doctors told the BBC.
The gunmen fired from rooftops overlooking the central square in what the opposition called a massacre.
President Ali Abdullah Saleh later declared a national state of emergency.
He said he regretted Friday's casualties but denied security forces had been behind the shooting, as the opposition demanded his resignation.
"There is no longer any possibility of mutual understanding with this regime and he has no choice but to surrender authority to the people," Yassin Noman, rotating president of Yemen's umbrella opposition group, was quoted by Reuters news agency as saying.
Another opposition spokesman, Mohammad al-Sabri, accused Mr Saleh of presiding over a "massacre".
"This is part of a criminal plan to kill off the protesters, and the president and his relatives are responsible for the bloodshed in Yemen today," he told the Associated Press news agency.
In international reaction, France demanded an end to attacks "by security forces and armed pro-government groups... against people exercising their rights to free speech and demonstration", Reuters reports.
