Hundreds of dissidents have been arrested across Syria, including in the flashpoint town of Deraa and a besieged Damascus suburb, after dozens were killed in weekend protests, activists say.
Looking for men under 40 years old, security forces broke into houses on Sunday in the old quarter of Deraa, witnesses said.
Prominent rights campaigners were also arrested in the eastern cities of Qamishli, Raqqa and in suburbs of Damascus.
Anti-government activists called for fresh protests on Sunday in a bid to break the week-long siege of the capital's Douma suburb and of Deraa, as well as in solidarity with other towns faced with deadly crackdowns.
Six civilians were killed in Deraa on Saturday, a day after massive protests rocked the country. Rights groups said the civilian death toll from unprecedented demonstrations that erupted on March 15 has topped 580.
David Cameron, the British prime minister, denounced Syria's "disgraceful" crackdown and urged more global pressure against Damascus, although Ahmet Davutoglu, the Turkish foreign minister, warned against foreign military intervention.
The United States has blocked assets of Syrian president Bashar al-Assad's brother Maher, who commands the feared Fourth Armoured Division, as well as top officials and Syria's intelligence services.
The European Union is preparing a raft of sanctions, including an arms embargo.
Despite the international condemnation, reports of arrests continued.
"At least 356 people were arrested today across Syria, including in Deraa, Douma, Latakia and Qamishli," an activist told AFP by telephone.
He said demonstrations took place in the central city of Homs, where protesters chanted "the people demand the fall of the regime" in the Khaldiyeh area near Nur mosque.
Syrian forces also arrested a prominent human rights lawyer in the city of Raqqa after he criticised
the authorities' reaction to the anti-government protests, the Syrian Observatory for Human Rights said.
The Observatory said Abdallah Khalil was arrested on Sunday by military intelligence agents at the Place of Justice in his home city of Raqqa in eastern Syria.
Khalil challenged president Assad in 2007 for running unopposed in a referendum that gave him a second term as president. He made the comments on Al Jazeera.
Security forces also arrested two opposition figures who had called for pro-democracy demonstrations in the northeastern city of Qamishli, a Kurdish source said on Sunday.
The source said Abdelqader al-Khaznawi and Abdelsamad Ali played a role in protests which have grown in strength in the region.
Meanwhile, a protest was also held in the coastal city of Latakia and a candlelit vigil in Baniyas.
In Deraa, troops in tanks backed by other armoured vehicles cruised the streets on Sunday, shooting to keep residents indoors and arresting men aged 15 and over, an activist from the town said.
Quoting a source in Deraa, Al Jazeera's Rula Amin said the situation in the city remained tense though tank shelling had ceased.
"Security forces are intensifying their house to house searches and (there have been) many many random arrests.
"He says hundreds of people have been arrested ... government says it's forces arrested 149 people in Deraa. Old city and the Karak neighbourhood are bearing the brunt of the operation today."
Douma under siege
In Douma, "the army has tightened the siege and has a list of 200 names of people it wants to arrest", another activist said.
A young man told AFP how he escaped on Sunday from Douma at "great risk" using back roads. "It was very difficult and very dangerous. I had to avoid all the checkpoints."
"People are afraid to leave their homes but they are beginning to feel the pinch. There are shortages of food, baby milk and other basic stuff," he said.
In Deraa, "nothing is coming in. People there grow a lot of tomatoes and zucchini and they are making do with what they have. The situation is critical but there is no famine."
The Syrian Revolution 2011 Facebook page, a driving force behind the protests, vowed in a statement that "we will only kneel before God," and gave a daily schedule of protests for the week in solidarity with Deraa and Douma.
The army said it entered Deraa on April 25 at the request of residents to rid them of "terrorist gangs" responsible for a spate of killings and vandalism.
"The manhunt for terrorist groups has led to the death of six of them and the arrest of 149 wanted people as well as the seizing of a quantity of arms," according to a military spokesman.
He said one soldier was killed and seven others wounded on Saturday in Deraa, where water and power have been cut since between 3,000 and 5,000 troops backed by tanks stormed the town at dawn last Monday.