Activists says Syrian troops have arrested hundreds in the central city of Hama, site of huge protests last week against President Bashar al-Assad.
A local doctor said three people also were killed, including a 13-year-old boy.
Residents said at least 30 buses carrying soldiers entered Hama in the morning on July 4.
Troops then searched homes, carried out arrests, and, residents say, opened fire in some cases.
Ramid Abdelrahman, president of the Syrian Observatory for Human Rights, told Reuters that 250 people had been arrested in Hama neighborhoods.
Reports say hundreds of youths have used garbage containers and other material to block roads leading to the city's central residential neighborhoods to try to prevent a possible advance.
Activists say one month ago security forces killed at least 60 protesters in Hama.
After that security was eased in the city until July 1, when at least 150,000 took to the streets to demand Assad step down.
On July 2, Assad fired the provincial governor and on July 4 residents say troops and police entered Hama to carry out arrests.
Rights groups say some 1,300 civilians have died in the 14-week uprising. Syrian authorities say 500 police and soldiers have been killed by gunmen who officials blame for most of the violence.
The United States and European Union have slapped sanctions on Assad and his inner circle over the crackdown.
compiled from agency reports