The military had largely stayed out of Albokamal, and Deir el-Zour, another town in northeastern Syria that is near Iraq, since antigovernment protests began in mid-March. Activists say the military has remained outside Albokamal because of fear that its presence there could ignite the anger of the region’s tribes.

The tribes wield great influence and have extended relations with tribes in the Iraqi province of Anbar, which endured some of the worst violence during Iraq’s sectarian insurgency.

Although Sunni Muslims make up the majority of Syria’s population, the Assad clan, which is part of the minority Alawite sect, has governed for four decades.

“I expect the regime to send more troops to seize the city and punish those soldiers who defected,” said a resident of Albokamal who arrived in Damascus on Sunday. “It will be a big mistake to let the army enter our city. We want democratic peaceful change, not a civil war.”

The troops arrived a day after the security forces and armed men in plain clothes killed five protesters in the town, including a 14-year-old boy. The killings angered residents, bringing thousands into the streets and overwhelming the security forces.

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Residents said soldiers from at least four armored vehicles joined the popular uprising, along with dozens of personnel from Syria’s Air Force. Their accounts could not be independently verified, because the government has barred foreign journalists from entering the country.

A video uploaded to YouTube, which was said to be from Albokamal, showed residents standing on two tanks and an armored personnel carrier and chanting, “The people and the army are one hand.” The slogan was widely heard during the Egyptian uprising that toppled President Hosni Mubarak in February.

Also on Sunday, Syrian security forces stormed Zabadani, a town 25 miles northwest of Damascus and near the Lebanese border, cutting electricity, phone lines and Internet connections, activists said.

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Mr. Assad has dispatched troops to almost every corner of the country to crush the uprising, but protesters say discontent against his government has continued to grow gradually. On Friday, the largest protests yet were held across the country, and activists said that on Saturday at least 30 people were killed and 500 were arrested.

“Friday’s demonstrations shocked the regime,” an activist in Damascus said. “The regime has no other choice but to kill and arrest opposition figures.”

Human rights activists said that the crackdown on protesters has killed at least 1,400 protesters since March and that more than 12,000 are in detention, most of them without being charged.

The government disputes the estimate on the number of protesters who have died. In addition, it blames Islamist extremists for the unrest, accusing them of killing hundreds of soldiers and other security forces.

On Sunday the authorities also arrested Ali Abdullah, a prominent opposition figure, at his house in a Damascus suburb, according to his son, Mohammad, a pro-democracy activist. Mr. Abdullah, who spent the last four years in prison, was released on May 30 when Mr. Assad issued an amnesty.

The military operations outside Albokamal came a day after activists and opposition figures met in Turkey and formed a united front against Mr. Assad. The president has been in power since 2000, when he inherited the post after the death of his father, Hafez, who ruled for three decades with an iron fist.