Syrian soldiers rolled into flashpoint cities in tanks and set up barriers topped with machine guns as President Bashar Assad's deadly crackdown on dissent pulled the country deeper into international isolation.
On the eve of another round of large protests, US secretary of state Hillary Clinton slammed the government's assault on demonstrators and said the violence showed Assad was weak. But she stopped short of saying he must quit.
"Treating one's own people in this way is in fact a sign of remarkable weakness," Mrs Clinton said during a trip to Greenland.
And Australian foreign minister Kevin Rudd said his government was stepping up targeted financial sanctions against key regime figures responsible for human rights abuses and imposing an embargo on arms and other equipment used for internal repression.
Assad, 45, is determined to crush the two-month-old uprising despite international pressure and sanctions from Europe and the US. His government has led one of the most brutal crackdowns in the wave of popular revolts sweeping the Arab world.
Protests organisers were calling for more demonstrations Friday despite military operations and arrest raids meant to pre-empt the rallies.
"Authorities are detaining any person who might demonstrate," Rami Abdul-Rahman, director of the Syrian Observatory for Human Rights, said.
In the northern city of Deir el-Zor, authorities placed cameras inside and outside the Osman bin Afan mosque, where many worshippers have been demonstrating after Friday prayer services, he said.
Mr Abdul-Rahman added that many former detainees were forced to sign documents saying that they were not subjected to torture and would not take part in future "riots".
A Western diplomat said 2,000 people had been detailed over the past two weeks, with a total of around 8,000 since the Syrian government launched its crackdown.
The official, who demanded anonymity to share assessments of the situation in Syria, said Western nations believed that between 600 and 800 people have been killed so far.
A video dated April 27 in the southern city of Daraa emerged yesterday showing a sniper shooting two people on a motorbike, then preventing residents from rescuing them or getting close.
People in the street could be heard screaming "Traitors!" at the security forces.
Syrian soldiers and tanks surrounded the city of Hama, to which President Assad's father had laid waste in 1982 to stamp out an earlier uprising, an activist said. Forces also used clubs to disperse 2,000 demonstrators on a northern university campus on Wednesday night.
In the central city of Homs, a resident said soldiers set up sand barriers with machine guns perched on top. He added that three tanks were still in the area, despite a report by the private daily Al-Watan that said the army has pulled out of the city after completing its mission.
"It seems they are getting ready for tomorrow," the resident said.
Most witnesses spoke on condition that their names not be published out of fear for their personal safety. The government has imposed a media blackout, refusing to let most journalists in and restricting access to troublespots.
The revolt was sparked in mid-March by the arrest of teenagers who scrawled anti-regime graffiti on a wall. Since then, the protests have spread nationwide and the death toll already has exceeded those seen during the uprisings in Yemen and Tunisia.
The government's bloody crackdown has increased in intensity in recent days. The army shelled residential areas in central and southern Syria on Wednesday, killing 19 people, a human rights group said.
The shelling of neighbourhoods evoked memories of Assad's father and predecessor, Hafez, whose most notorious act was the shelling of Hama in 1982.
He levelled the city to crush a Sunni uprising there, killing 10,000 to 25,000 people, according to Amnesty International estimates. Conflicting figures exist and Syria has made no official estimate.