Mohammed al-Qubati, the director of a field hospital at the main protest site in Sanaa dubbed "Change Square," said more than 70 protesters were injured in the protests demanding the resignation of President Ali Abdullah Saleh.
Tens of thousands of protesters marched through Sanaa, led by shirtless young men with the words "Leave ... you butcher" scrawled across their chests, referring to Saleh.
Soldiers from the Republican Guard, a loyalist unit led by Saleh's son Ahmed, arrested four female protesters who were ahead of the main demonstration, said activist Habib al-Uraiqi.
Abdel-Rahman Berman of Yemen's National Organization for Defending Rights and Freedoms (HOOD) said Saleh's forces used live ammunition and harsh tear gas.
Berman said HOOD team monitoring the situation charged that government forces and thugs abducted female protesters and some wounded demonstrators in a "shameful and criminal way."
Similar demonstrations were held in other parts of Yemen, including the southern cities of Aden and Taiz, protest organisers said.
The protesters called for Saleh to be put on trial for killing demonstrators and urged the international community and the UN Security Council to help topple him.
Today, key members of the Security Council began considering a British-drafted resolution that would call for an immediate cease-fire in Yemen and transfer of power, as well as immediate action by Yemeni authorities to end attacks against civilians. The consultations are still in progress.
President Saleh is accused by many Yemenis of pushing the country into civil war by tenaciously clinging to power in the face of eight months of mass protests across the country, the defection to the opposition of key tribal and military allies and mounting international pressure on him to step down.