(CNN) -- Violence raged Friday in Syria as anti-government protests unfolded across the restive country, but Hama appeared to have borne the brunt of it.

The city endured steady shelling and bombing Friday morning while the government's military offensive continued in full swing, said a resident whom CNN has not identified for his safety.

Two witnesses, who talked to CNN independently, said scores of people -- one said he had counted 53; the other said 58 -- were killed instantly when a tank positioned 150 meters from Hurani Hospital launched an artillery shell that landed in front of the building.

The incident occurred after a number of demonstrators had come to the aid of two others

who had been wounded by gunfire from Syrian soldiers, a witness said.

Pharmacy shelves were bare in Hama, and Syrian army and security forces were occupying city hospitals, thereby preventing the wounded from receiving treatment, a witness told CNN. As a result, some people died from wounds that would not otherwise have proved fatal, he said.

One opposition activist estimated that 300 people have died in violence in Hama during the past six days. Mourners were unable to bury their dead because the cemeteries are outside the city, and the military controls access, the activist said. Instead, "they dug holes in their backyards and buried the bodies," he said.

Syrian state TV, which aired video of a tank and rubble on deserted Hama streets Friday, reported that the army had carried out an operation "to restore the peace of mind to the families" of Hama.

The state-run Syrian Arab News Agency has said "armed terrorist groups perpetrated acts of sabotage and killing through setting up barricades, breaking off roads, attacking and burning police stations using different kinds of weapons."

Compounding the devastation is the mounting humanitarian emergency, the resident said, citing a lack of electricity and rotting food.

"So now we don't have food to eat," he said. "We just are fasting."

Hama was the site of the bloody 1982 crackdown by the Alawite-dominated government against a Muslim Brotherhood uprising. Memories of that siege, carried out by the late President Hafez al-Assad, the current president's father, have reverberated across the country and in the city.

Elsewhere, opposition members said, thousands kicked off marches in cities and towns across the country after Muslim prayers, as they have every Friday for weeks, demanding political reforms or the fall of the regime and railing against the violent crackdown directed at peaceful protesters.

Each Friday, nationwide protests have had a theme. This week's was "God is with us."

In the Bab Amr neighborhood of Homs, clashes broke out Friday between Syrian security forces and army defectors who had refused to obey orders to fire at fellow Syrians, a witness said. Two people were killed and a number of others were wounded, he said.

The Syrian Observatory for Human Rights, an activist group, put Friday's death toll in Syria -- not including Hama -- at 14. But a spokesman for the Local Coordination Committees cited 21 deaths -- not including Hama. They broke down as follows: seven in the town of Arbeen; six in Homs; three in Doumayr; two in the Damascus suburbs of Moadamiya and Nahr Ayshi; one each in Daraa, Douma and Qaboon.

The observatory said tanks were in the northeastern city of Deir Ezzor. There, tribal men were bracing for an assault by the military, according to a video posted on YouTube. The video shows a man telling tribal leaders that he had received information that the army would conduct an operation and asking the men to prepare.

More than 2,000 people, the vast majority of them protesters, have died since anti-government demonstrations erupted nearly five months ago, the observatory said. Activists blame the deaths of demonstrators on security forces, but the Syrian government has consistently attributed the violence to "armed groups."

U.S. Secretary of State Hillary Clinton said President Bashar al-Assad has lost legitimacy as a leader and noted the mounting death toll:

"We think, to date, the government is responsible for the deaths of more than 2,000 people of all ages."

CNN is unable to independently confirm death tolls or events in Syria, which has restricted access to the country by international journalists.

Also Friday, the U.S. Department of State urged U.S. citizens in Syria to depart immediately while commercial transportation is available.

"Given the ongoing uncertainty and volatility of the current situation, U.S. citizens who must remain in Syria are advised to limit nonessential travel within the country," it said.

The U.S. ambassador to Syria, Robert Ford, returned Friday to Syria to continue his duties.

"He will remain engaged with the Syrian government to make clear that the Assad regime's brutal repression of the Syrian people must cease immediately," State Department acting spokesman Mark Toner said.

CNN's Arwa Damon, Nada Husseini, Kamal Ghattas, Salma Abdelaziz, Tracy Doueiry and Jack Maddox contributed to this report.