Forces loyal to President Bashar al-Assad shot dead 14 people on Tuesday in the Syrian city of Hama, activists said.
Tanks were still surrounding Hama, days after it witnessed some of the biggest protests against Assad's rule since a 14-week uprising erupted in March.
The attacks focused on two districts north of the Orontes River, which splits the city of 650,000 people in half. Residents said the dead included two brothers, Baha and Khaled al-Nahar, who were killed at a roundabout.
Troops raided towns to the northwest of Hama near the border with Turkey in Idlib province, and authorities intensified a campaign of arrests that has resulted in the detention of at least 500 people across Syria in the last few days, rights campaigners said.
In the eastern provincial capital of Deir al-Zor, security forces arrested Ahmad Tuma, a former political prisoner and secretary general of the Damascus Declaration, a grouping of opposition figures founded in 2005 to unify efforts to transform the country into a democracy.
"Heavily armed 'amn' (security police) came to Dr Tuma's clinic and dragged him away in front of his patients," one of Tuma's friends told Reuters by phone.
Some residents of Hama, scene of a crackdown by Assad's father nearly 30 years ago, had sought to halt any military advance by blocking roads between neighbourhoods with garbage containers, burning tyres, wood and metal.
Tuesday's raid by security forces and gunmen loyal to Assad followed the killings of at least three people when troops and security police entered Hama at dawn on Monday.
"Assad may wait to see whether large-scale protests in Hama continue. He knows that using military aggression against peaceful demonstrations in a symbolic place like Hama would lose him support even from Russia and China," Syrian activist Mohammad Abdallah told Reuters from exile in Washington.
Abdallah said using tanks to attack Hama would "totally discredit" a promise made by Assad to seek dialogue with his opponents. Troops and armour were attacking villages and towns in the Jabal-al-Zawya region, north of Hama, which had been the scene of large protests against Assad's 11-year rule, he said.
Assad's father, Hafez al-Assad, who ruled Syria for 30 years until his death in 2000, sent troops into Hama in 1982 to crush an Islamist-led uprising in the city where the Fighting Vanguard, the armed wing of the Muslim Brotherhood, made its last stand.
That attack killed many thousands, possibly up to 30,000, and one slogan shouted by Hama protesters in recent weeks was "Damn your soul, Hafez".
Authorities have prevented most independent media from operating in Syria, making it difficult to verify accounts from activists and authorities.
Rights groups say Syrian security forces have shot and killed at least 1,300 civilians across the country since the protests started and arrested over 12,000.
Several troops and police officers have been killed for refusing to fire at civilians.
Authorities say 500 police and soldiers have been killed by gunmen, who they blame for most civilian deaths.
Assad has promised a national dialogue with the opposition to discuss political reform in Syria, which has been under the iron rule of the Baath Party for nearly 50 years. Many opposition figures reject dialogue while the killings and arrests continue.
Güncelleme Tarihi: 06 Temmuz 2011, 13:15