Syrian troops and security forces have set up checkpoints across the country amid new reports of shootings and mass arrests of opposition supporters.
Activists said 120 people died in the weekend's violence, with funerals taking place on Sunday for mourners who were killed on Saturday.
Evidence of mounting repression by President Bashar al-Assad brought public condemnation from Britain and a demand by Human Rights Watch for punitive sanctions to be imposed on the Damascus regime.
The Foreign Office warned Britons to get out of Syria while commercial flights are still available, unless they have a pressing need to remain in the country, as the security situation is "rapidly" deteriorating.
Calls for Assad's overthrow were heard at a funeral attended by thousands in the southern town of Nawa, where four people were killed.
Despite an official media blackout, amateur film footage from an earlier funeral showed mourners bearing coffins being met by hails of bullets. An al-Jazeera journalist described people "lying flat on the road, taking cover behind those who had already been wounded or shot dead" in Ezraa.
In Jabla, a coastal town close to Latakia, locals reported security forces and members of a gang loyal to the Assad family shooting dead two people after a small protest. "We are scared, very scared," a shopkeeper told the Guardian by telephone.
The official violence was an ominous sign that despite making belated political concessions to protesters, including abrogating Syria's decades-old state of emergency, the regime appears bent on a harsh response.
In Barzeh, a suburb of Damascus, sheikhs in mosques begged on loudspeakers for security forces to stop firing and called for medical help. Easter parades were cancelled but the centre of the capital remained lively with Christian families in their Sunday best and children playing after church services.
Thousands took to the street in nearby Duma to bury Bilal Shihab, Ahmad Shihab Ma'rawi and Khaled Sedawi, three of at least 12 shot people dead across the country by security forces on Saturday – bringing the weekend's death toll to more than 120. Syrian state TV said nine members of the security forces were killed, seven in clashes with "armed gangs" in Nawa.
"It's starting to look like the West Bank," warned Radwan Ziadeh, the US-based head of the Damascus Centre for Human Rights who has contacts across the country.
A doctor from Moadamyeh, where four died on Friday, said anyone trying to leave the town – "even the injured" – was arrested at checkpoints. "I fear there will be mass arrests," he said. The Syrian Observatory for Human Rights said dozens were arrested after demonstrations on Friday, including nine in Idlib province, four in Syria's second city of Aleppo and five in eastern Raqqa province.
The international mood was hardening too. "The Syrian authorities must act quickly and decisively to calm this dangerous situation and can only do so by responding to the legitimate demands of the Syrian people," said William Hague, the foreign secretary. On Friday, Barack Obama urged Assad to stop the "outrageous use of violence to quell protests".
Many Syrians believe a turning point has been reached with the weekend violence a sign of the fight to come – and that they have a moral responsibility to take sides.
The regime appears to believe it can quash protests using the same methods it used against prior internal and external threats – such as in Hama in 1982, when at least 10,000 were killed when an armed Islamist uprising was crushed.
In 2001 the regime clamped down on debate which had flourished in the early days of Assad's rule – inherited in 2000 on the death of his father, Hafez – which was known as the Damascus Spring. Forums were closed, and many intellectuals were imprisoned. In 2005, after the assassination of former Lebanese prime minister Rafiq al-Hariri, blamed on Syria – a charge Damascus has always denied – Assad's regime stood firm against foreign threats whilst clamping down on dissidents inside.
"We have seen how the Ba'ath regime under Bashar acts when under threat. This explains why we are sceptical of promises of reform," said Mahmoud, a 35-year-old office worker in the capital.
Thousands of families, in a country where extended families are close, have been personally touched by the violence: several officials in the southern town of Deraa, where protests first broke out on 18 March, have defected, including Moammar Shuhadat, the director of endowments. Syrians are watching higher level officials from Deraa, including the vice-president, Farouk Sharaa.
Sheikh Ahmad al-Sayasna, the imam of Deraa's Omari mosque, said: "There is no negotiation with a government that kills our sons, there is no going back."
Syrian protesters hope that if violence escalates the army would take their side, as it did in Egypt. Some conscripts have reportedly been shot for defecting, but the upper echelons are exclusively populated by officers from Assad's minority Alawite sect, and others who have too much to lose.
Syria has a limited number of non-governmental and humanitarian organisations that can treat a growing number of wounded. "It's a bad situation," said the doctor in Moadamyeh. "We have limited ability to treat some of the patients here and we can't send them out."
