Witnesses: Security forces fire on Yemen protesters, killing 5

What's behind escalating Yemen violence?

(CNN) -- Yemeni security forces opened fired on demonstrators gathering for a planned march in Sanaa on Sunday, killing four people and injuring 54 others, according to a medic on the scene.

"The injured are entering the hospital by the minute. We need help. We call on people to donate blood for the injured," Mohammed Al-Qubati, who works at a field hospital in Change Square.

Five of the wounded are in critical condition he said from the square that has become the center of protests against President Ali Abdullah Saleh.

Protesters carried signs with slogans including: "Saleh kills and the world watches. Is this the justice the west preaches?" according to witnesses and activists.

Others carried flowers or signs that said: "We are not armed, Don't attack us with gunfire," witnesses told CNN Sunday.

Some chanted: "Oh Saleh the oppressor your time will come, our blood is not cheap."

Gunfire is still being heard in the square, people there said.

In a separate demonstration in the city of Taiz, a woman was killed by a shot to the head from a government sniper, medics and eyewitnesses in Freedom Square Taiz said.

Medics named her as Aziza Othman Kaleb, from the Maweah district of Taiz.

Four others who were next to her were also injured by the snipers in the march, the same sources said.

CNN has not independently confirmed the details of casualties in Sanaa or Taiz, and the government has not yet responded to CNN requests for comment.

The reported violence comes a day after at least 10 people were killed and 38 others wounded in clashes in the capital, Al-Qubati and others said.

Molhim Saeed, another medic in Change Square, called Saturday "a sad day for the revolution."

"The marches were peaceful and the youth were unarmed. They refused to even fight back when they were being shot at," Saeed said.

A State Department spokesman late Saturday urged President Ali Abdullah Saleh to hand over power.

"We are deeply concerned by recent violence in Yemen.  We extend our deepest sympathies to those who lost loved ones as result of this violence," said Aaron Snipe, of the State Department's Bureau of Near Eastern Affairs.

"All those wishing to express themselves must be allowed to do so peacefully.  We urge a prompt, impartial investigation into the events that led to the recent violence," he said. "We again urge President Saleh to initiate a full transfer of power without delay, allowing the Yemeni people to move down a path toward a unified, stable, secure, and democratic Yemen."

There was no immediate comment from the government. Demonstrators have taken to the streets regularly to call for an end to Saleh's rule, which began more than three decades ago.

Last week, tens of thousands of protesters gathered in Sanaa, marching, chanting and calling for the United Nations to come out with a firm resolution in support for change in the country.

For its part, the government says it is trying to come up with a solution to end the political stalemate.

"The ruling party is serious on finding a solution to the political crisis from its roots to ensure they don't erupt in the future," said Tareq Shami, spokesman for the ruling party, the General People's Congress.

Saleh is also facing armed revolt from members of powerful tribes in his country.

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