(CNN) -- As nightfall arrived Monday in Syria, many residents of Homs went to bed afraid that a historic siege on their city could be imminent.

Opposition figures said the Syrian government had warned people in Homs to stop anti-government protests, hand in weapons and surrender defecting military members by Monday night -- or face attack by the government forces.

Syrian forces gave a 72-hour warning, said Lt. Col. Mohamed Hamdo of the Free Syrian Army, an opposition group of defected Syrian military personnel. Activists on the ground said the ultimatum was issued Friday for Homs, a center of the popular uprising.

Hamdo said Syrians are worried about a repeat of what happened in 1982 when Syria's military -- acting under orders from then-President Hafez al-Assad, father of current Syrian leader Bashar al-Assad -- launched an assault on Hama, killing thousands. "We fear that a similar massacre or worse could take place in Homs," he said.

"People are very afraid," said Wissam Tarif, a human rights activist in Beirut, Lebanon, with the organization Avaaz, who is in touch with people in Syria.

There are enough troops around Homs "to take over the city," he said, and casualties have been increasing "in very big numbers" over the past couple of days. "People are afraid that the army might now invade the city."

Hamdo said the military has dug trenches around Homs and largely cut it off.

"The situation in Homs is really bad. There is no electricity, water, and the communication lines are much worse. The food supply is also decreasing, mainly because little food is going in," he said.

The Syrian National Council, the country's leading opposition movement, earlier warned of a potential bloodbath in Homs at the hands of the Syrian regime.

The Syrian government denied reports of water and electricity being out in the city, according to the state-run Syrian Arab News Agency.

The government has not acknowledged a deadline for Homs on state-run media.

The Local Coordination Committees of Syria, a network of opposition activists in the country, said Monday the Syrian army and security forces killed 21 people, including four women and three children. Thirteen of the deaths were in Homs, three were in Hama, three in Damascus suburbs and two in Idlib.

Meanwhile, state TV painted a picture of normalcy, with reports of local elections under way across the country Monday.

SANA noted that more than 3,000 candidates are vying for seats in the Homs region alone. It billed the elections as part of the "process of building institutions, promoting democracy and achieving the comprehensive reform process led by President Bashar al-Assad."

But there were reports of violence across Syria on Monday.

Fierce clashes broke out between security forces and defectors in the cities of Daraa and Idlib, according to the London-based Syrian Observatory for Human Rights. The organization collects information from people in different parts of the country.

Hamdo of the Free Syrian Army said, "We conducted an operation late last night against the Syrian forces in Idlib and killed eight of them and injured 22. Two of our men are critically injured. "

Syrian forces were conducting mass arrests of shop owners who shut their stores Sunday as part of a nationwide anti-government strike, Hamdo said.

Via SANA, the Syrian government on Sunday quoted people saying there was no strike and no sign of a strike.

The Local Coordination Committees of Syria said gunfire and tanks are centered on checkpoints in Daraa.

SANA reported that "seven army, security and police martyrs" killed "by armed terrorist groups" in Homs, Hama, the Damascus countryside and Daraa were taken "to their final resting place in their hometowns and villages."

SANA also said "competent authorities" in Daraa province killed four gunmen and wounded others who were attempting "to influence the local elections atmosphere and spread tension."

Throughout the uprising, Syria has insisted it is not targeting peaceful protesters and instead cracking down on armed terrorist gangs.

The state-run news agency also accused "an armed terrorist group" of killing the director of a gas station in Homs on Sunday.

"Meanwhile, the authorities in Homs who hunt the terrorist groups stormed one of the dens of the armed men at al-Areda Village in Tal Kalakh, killing one of them, arresting others and confiscating their weapons," SANA said, adding the "terrorists" had targeted law enforcement.

SANA also reported instances of authorities clashing with gangs and killing some "terrorists" in other cities.

Reports of deaths between demonstrators and government forces have escalated over nine months as protesters demand democratic elections and the end of al-Assad's regime. Al-Assad has been in power since 2000; his father, Hafez, ruled Syria for three decades.

The United Nations said this month that more than 4,000 people have died in Syria since the crackdown began in mid-March.

CNN cannot independently confirm events because the Syrian government restricts access of international media to the country.

The Arab League announced it will hold emergency meetings this week in Cairo. In a statement on Egypt's state-run MENA news agency, an Arab League official said leaders will "discuss the Arab response to a message from Syrian Foreign Minister Walid Moallem to approve the signing of an agreement on an Arab League observing mission to Syria with conditions."

World leaders have widely condemned Syria's crackdown and called on it to halt violence against the opposition.

On Saturday, France expressed its concerns, warning Syria about launching a military operation against Homs and its population.

Israeli President Shimon Peres on Sunday described Syria's president as a "killer," implicitly comparing him to Libya's Moammar Gadhafi.

"The world decided -- including the Arab world -- to intervene when a leader is beginning to kill his own citizens," Peres told CNN. "It happened in Libya; it's happening in Yemen; it's happening by the Arab League, for the first time in their experience. They decided to put pressure on an Arab state because the leader is killing his people."

CNN's Hamdi Alkhshali, Amir Ahmed, Josh Levs and Richard Quest and journalist Mohamed Fadel Fahmy contributed to this report.