Homs has been surrounded by security forces and troops since mid-March, when the popular uprising against President Bashar Assad began, inspired by uprisings in Egypt and Tunisia. But Thursday's operations were, by the accounts of many protesters, the largest in Homs.

The military campaign came five days after a wave of sectarian killings in the city that threatened to provoke wider conflict. Although most Syrians are Sunni Muslims, there are a number of sizable religious and ethnic minorities, and Assad and his ruling clan belong to the minority Alawite sect.

"Residents are very scared and worried," said an activist in Homs who gave only his first name, Abdallah. "There is random shooting, burning and destroying buildings, and a lot of arrests and raids."

He said that neighborhoods had been sealed off and that people were trying to build barricades in the streets to block raids by security forces. Other residents spoke of shortages of bread and other staples.

A spokesman for the Local Coordination Committee, an umbrella group that helps organize and document protests across Syria, said from the committee's base in Beirut that at least one person had been killed in the military operation in Homs and that 150 people between the ages of 15 and 50 had been arrested.

In all, human rights activists and residents of Homs said, at least two dozen people have been killed in the city since Saturday.

Human rights groups said that across Syria, at least 1,400 people have been killed since the demonstrations against Assad began four months ago. The government disputes that figure and says extremists are to blame for the unrest.