Recent amateur video from the southern city of Taiz in Yemen showed forces loyal to the country's embattled president attacking protesters who were calling for his resignation.
Soldiers fired bullets and also used tear gas and water cannons.
A leading Yemeni activist said dozens of demonstrators had gunshot wounds. In three protests nationwide, two people were reported dead.
Since Yemen's unrest erupted more than two months ago, at least 130 people have been killed by security forces and supporters of President Abdullah Ali Saleh.
Saleh has ruled Yemen for 32 years.
Sunday in San Francisco, Calif., hundreds of people rallied at the United Nations Plaza to show their solidarity with protestors.
"People in Yemen are being killed for nothing more than standing up and speaking out against injustice," one attendee said. "They have been protesting peacefully. We demand the United States government end all military aid to Yemen."
There were reports that Saleh agreed to an Arab proposal to leave within 30 days of signing a deal and transferring power to a deputy, however that agreement could now be off.